Let's take a breath and imagine the world without the computers and train simulation, just looking back few decades ago, probably earlier than the diesel era. We all remember a lot of stories how the steam locomotives were made in different countries. We also probably remember by heart the names of the locomotives, the factories and even the names of the developers (I can even say one, my grand-grand-father, just because he made edits in steam bluprints, when he worked in steam building works as an engineer).

But. What do we really know about the fundamentals of the steam locomotives, the physic principles, that make them move? Probably not everyone can answer, just because this topic very likely was not discussed in the school, colledge and even university courses because it's now abandoned and outdated. But, even now, some people are trying to invent the steam wheels just to run them in their garden railroad.

So to understand what it is, we must go deeper to the details of the steam locomotives design, and take a look to main parts of it:

• the pistons
  
• the boiler and the chimney
  
• the steam-distribution mechanisms
  
• the brake system

Considering that this manual may be boring or technically overweighted, you can feel free to skip some sections and return to them little bit later just if you want. Also I added special chapters:

• differences between steam and other types of traction
  
• how the steam locomotives move

To give a theoretical understanding on how steam locomotives go from full stop to max speed and what makes them so different from the other locomotives in real life while at the same time having many features in common.

Let's begin from the main part of the steam locomotive - the pistons. What are they and what is the difference between them and pistons of combustion engines or other mechanisms?

The fundamental difference is that they utilize the steam, that isn't burnable (like the diesel, petrol or gas) and is compressable (the fluids in hydraulic machines aren't). So to express how the piston works we will look at one analogue in real world.

The closest analogue is bycicle pump. It does not utilize steam, but behaves very similar, but like the "reversed" steam machine.

When you push the pump, the air compresses and gets warmer. In contrast, in steam locomotives, the pressure decreases when the piston is pushing forwards and temperature going lower, because it is converted to mechanical energy to rotate the driving gears of the steam machines (I specially added Steam Logo in the beginning of this manual to give the vision what is happening in short).

When you start pumping the bicycle pump, you start from lower speeds to increase the pressure on early stages. As far, as pressure (resistance) increases, you start to make faster and shorter strokes to maintain bigger pressure. Ideally, to achieve the needed pressure, you just make series of short but powerful strokes that need from you much more power and force (the steam locomotives can do that only going without the consist).

And now we stopped here. If the bicycle pump is analogue of the steam piston, why the length of stroke is always fixed? It's because we can't easily disconnect or shorten the driving gears (bars) and pistons just because nearly the half of train weight (hundrends or thousands of tons) or traction power (if running single head two-piston locomotive) is applied to the each driving gear, so instead of shortening the piston stroke, we just shorten the portion of steam, changing the steam valves opening and closing timings or decreasing the pressure by closing the throttle - and here the steam pistons are similar to pistons of the cars, sea vessels, motorcycles and diesel locomotives. But, speaking about the changing the angle (what is very wide topic), the first steam machines (like the industrial water pumps and even first steam electric power generators) did not have the angle regulators, only the throttle.

To remember this just keep in mind simple model: force is defined only by portion of steam (pressure distributed in constant volume of cylinder). In reality you can meet a back force (when it's a lot of steam filled in so you waste the power to compress this extra portion of steam), but generally this formula works. In general it can be compared with the back pressure of bicycle air camera to the bicycle pump, when reaching closer and closer to the needed pressure.

The boiler and chimney are very important parts of steam locomotive and actually they both form a "power plant" analogue of diesel-electric or diesel-hydraulic locomotive. The main purpose of them to generate steam from water using the heat of burned fuel, that is later transmitted via pipes to the the pistons and other equipment (injectors, blowers, vaccum pump, compound-pump, the dynamo, the oilers and other systems).

How to understand how it works? Well, consider you have a steel kettle placed on the heating surface (gas, electric furnace or open fire) that both form the analogue of steam boiler and chimney. When you just put it on fire, you have cold metal and cold water. But after a while (depends of his size), it starts generating steam. And here starts the magic.

In the kettle your water can easily escape from the cover or whistle, so the water there consists of hot water with bubbles of steam and mixture of air and steam.

In the steam locomotives, in contrary, there are much more stages of steam of water, than in the kettle: the fluid, the boiling point, the saturated (wet) steam and the overheated (dry) steam (generally speaking, all above the water level is the steam and all below - the water).

And here comes another difference - the water level in kettle may always go down (and even safely boil out), but in the steam boiler the water may stay always in certain levels and in some circumstances may appear from nowhere (when the boiling temp become too high so the saturated steam condenses). You even can notice the situation of drastique decrease of water or steam levels if balance of steam consumption and generation was not met.

Other distinctive feature that make the boiler very different from kettle is the boiler construction itself. If you're familiar with the construction of nuclear reactor (at least it must be studied at school), you can notice the similarities of steam boiler and nuclear reactor active or nuclear fusion zone. The fuel tablets are placed in refillable fuel batteries, that fill the gaps in the active zone and the water entering the reactor cools the fuel batteries, that dissipate the heat generated by fusion of uranium or other radioactive element.

Now going back to the steam locomotive boiler and the chimney. The boiler has instead a heat pipes that dissipate heat from burned fuel and gives it to the water in the boiler. These tubes are directly connected to the chimney on the one part and the smokebox on the other part of the locomotive.

Like in the fusion reactor, the water then in steam stage (superheated or dry, depends on construction of the locomotive) is transmitted to the steam machines (like the steam turbine in the nuclear power plant, but acting differently) which is converted to mechanical power... The new portion of burned coal and fresh water comes into the boiler and heating pipes and the process begins again. The chimney is very similar to the chimneys of coal burning power plants and hearly indistinguishable. The main difference is the size and availability of automatic coal stockers or milled solid fuel injectors (similar to that you can see in oil-burning locomotives).

Usually due to the mode of non-constant steam usage, the steam boiler, comparing to the steam boiler in steam power plant issues more significant and fast changes of steam consumption and water levels. So the steam locomotive boiler (and the steam machines as well) allow you to take the various loads (even bigger than it can but in very short amounts of time), and the steam power plant don't (it ends with major damage to turbine, electric generator and even the boiler), but the main feature stay common in both cases - the heat source (boiler chest and heat pipes in steam locomotive or the power station) must be always cooled with water.

Just because this chapter is too precise and can't be uncertain or abstract, I'll change the material and format of this paragraph, adding self-explanatory videos from the YouTube, where are described the real life steam locomotives and other steam machines or their models. Also this chapter will have a numbered pargraphs, related to specific parts of the steam locomotives steam distribution mechanisms.

3.1. Steam distribution stages:
As it said in the previous chapter, related to the pistons, the steam locomotive has an option of "shortening" the stroke of pistons using the means of the steam pressure regulation and also the cutoff (reverser) regulator. In general, the throttle is nearly similar to throttle of diesel locomotives - it regulates the volume of steam entering the cylinders. In starting mode state, the cutoff is set to maximum and connected via system of driving and valve gears and bars to the pistons and don't affect the valve opening angles. Lets consider that the piston stays in bottom dead point (closer the first driving wheel). When the valve opens, steam enters the cylinder, the piston begin to move in the opposite direction (related to the the part of the piston that was pushing by the steam portion and reverser direction - forward or backward), the driving wheel begin to make revolution. Nearly after 90 degrees (on the middle of stroke), the valve of second piston opens from the opposite part and steam enters the second cilynder. Both cylinders now moving in opposite directions. After the 90 degrees (of 180 degrees of revolution from the start of the cycle), the steam in the first cylinder is now fully pushed the the piston to the upper dead point (near to the buffer lights) and the cylinder valve opens the output, giving the steam ability to exit the cylinder freely, because it has at least some pressure (depends on locomotive systems, this pressure may be used to increase the flow of the hot gases through the boiler using the steam cone hot air thrust, disposed directly to the tracks or used to move low pressure pistons in so-called "compound" designs, similar to high-low pressure turbine designs in steam power stations). Now, at the same time, (on 180 degrees), in the opposite part of the first cylinder, starts the same process as was 180 degrees ago, but in the opposite direction (steam enters to the front part of the first cylinder and pushes the the piston in the opposite direction). Just because the output port of opposite part (back) of the first cylinder is connected with the atmosphere, the pressure in the front part of the cylinder and its moving backwards forces the steam to exit the cylinder. After the 90 degrees of revolution (270 degrees from the beginning), the piston in the second cylinder stays in the same position as in the first cylinder 270 degrees ago. In the same time starts the stage of refilling of the back part of the cylinder and freeing the steam from the front part. After the 90 degrees (one full revolution) the process starts from the beginning.
So underscoring all this, we can say that in general this process is very similar to gas-distribution phases of the 2-stroke 4-cylinder opposite engine (1stroke - filling with fuel, ignition and work stroke, 2nd stroke - ventilation, refilling and combustion of the air), where opposite pistons depict the opposite parts of the steam pistons.

3.2. Steam distribution mechanisms:
According to the Wikipedia there were a lot of types of steam distribution mechanisms, but most of them are based on the Stephensons design. The main differences are mostly the movement type of these mechanisms, the bars and gears and the valve construction itself. Generally speaking, all them act as levers, repeating the movement of the driving gears, but having variable ratio (from 0:1 to ~1:1 ratio, but usually smaller ~ 75:100 to avoid the locomotive get stuck at some ratios when starting from full stop). Changing the lenght of this lever, the driver may change the amplitude (and direction if he wants to drive in opposite direction) of steam distribution valve movement (or rotation in case of rotary fast-acting valves in late designs) and thus the relative angle (duration) between the cylinder filling and its closing.

Lets' see main differences of three common types of steam distribution mechanisms.
The Stephensons link
Unfortunately I've found the good explanatory examples only of marine versions of this type of distribution mechanism. The main distinguish feature is orientation of "dividing" mechanism, that has focus of arc, oriented in direction, opposite to the cylinders.

The Walschaerts system
In contrary to Stephensons, the "dividing" mechanism has focus of arc, oriented in direction of the cylinders. Some designs (as on the first video) may also contain rotating valve, similar to Baker and Franklin systems.

Poppet valve systems
The main distinguishable feature is absence of piston valve above the pistons, nearly not noticeable "divider", and in contrary, the presence of rotating (poppet) valve above the valve gear only

In early era of steam railroading, a lot of train cars didn't have even manual brakes, so the trains stopped using inertia (compression) brakes and manual brakes at the braking vans or cabooses. If you remember how act the retarders and the compression brakes in heavy freight trucks - thats the same. The steam portion or air (depends on previous state of throttle and cylinder drain cox) begin to act as an air suspension in bike and tend the pistons to compress the air or steam whether they are moving in forward or backward direction. Usually this type of brakes is very limited in use because it uncreases the wear of the steam machines (the oiling in combustion engines works even when shuted down the throttle). Sometimes, a very little amounts in reverser were used to do this work in real locomotives, but it may cause entering the smoke into the cylinders that required the usage of special devices that shut down the thrust of hot smoke.

The first brakes ever made were steam direct brakes, that similar to modern locomotive brakes in action. The pressure (of compressed air or steam) moved the pistons in the brake cylinders forward and pushed the brake pads to the tires of wheels. Later this system was improved to use in passenger and freight trains.

After the invention of train brakes, they went two different ways - the American (Westinghouse brakes) and British vacuum brakes. The main distinguishable feature was the type of action. British ones act as direct brakes by appliying negative pressure (they do not have triple valve as the modern indirect brakes), that force the brake cylinders to release the brake pads. So the change of vacuum to normal pressure (just in case if train couplings between cars are broken) equals to applying emergency. In order to maintain pressure, enough to release the brakes, the steam locomotives had special device, named vacuum pump, that utilized the steam to pump out all the air from the brake pipe and the vacuum cylinders, the same way the steam used to blow the firebox or to inject fresh water in the boiler.

In contrary, the Westinghouse brakes used the positive (higher) pressure to act on brakes indirectly via brake pipe. So the pressure changes signalize to each triple valve in each car in consist to recharge all refillable air tanks in the cars and after the air brake pipe pressure increases. Just in case of train brake the pressure is lowered to some value, that immideatly causes the brake valves to increase the pressure in the brake cylinders by same value.
I've found one video on the YouTube describing fundamental differences between these two systems, unfortunately in Indian language, but the pictures are self explanatory.

Having the need of adding positive pressure made impossible to use the steam directly (sure, the steam can be used as source of pressure in such system, but due to corrosion in the sensitive brake triple valves the air is used instead). In order to maintain positive pressure, the locomotives had so-called compound-pump - a two stage air compressor with two pairs of pistons, air and steam inlets. When the steam entered the bottom part of one cylinder, the air was compressed at upper part. So sometimes one can be confused hearing this sound: it's somehow similar to the sound of steam machines exhaust.

Just because the steam locomotives appeared a long-long long ago and we all saw in many films, in westerns at least these black machines with moving gears and bars, we probably have a clue about them, but the fundamental differences are not that clear – in means of changing of traction effort, speed, momentum and even the efficiency of the power output. So just of educational purposes (it you are not already bored with bicycle, teapot and nuclear reactors), let’s compare steam traction with other types of traction.

Diesel-mechanic locomotives
The first locomotives and motive units were used petrol (and later diesel) engines as primary movers. The primary mover was connected via the system of gears to the axles that converted the rotation of the engine to the movement along the track. Quite simple, like the modern cars or trucks. And what was in common?
The common was the presence of pistons in the engine and possibility to change the gears. If you remember, the diesel or petrol combustion engines can change the throttle only and do not have any “cutoff”, but instead have a gearbox. That’s it. When the truck, car, or diesel-mechanic locomotive starts, it uses the lowest gear as possible and low throttle just to avoid a wheelslip. The same can be noticed in the steam locomotive – on the start the locomotive uses the full length of stroke to transmit the power from the steam into the movement of bars and gears. So the first gear and throttle may act the same way at the first while.
But after getting the speed bigger, the gears are changing and throttle is changed to lowest positions (if using manual gearbox having any means for synchronizing the speed of primary and secondary gears) and the traction effort falls down significantly. Does it matter? No, just because the steam locomotive has more “gears” – shorter the cutoff – “higher” is the gear. But in both cases we see the same – speed is bigger, the effort is lower.
What happens, when the gear is changed to lower “position” than needed on downhill or the throttle is closed on uphill? In both cases the steam locomotive and the diesel-mechanic railcar will lose their speed. In first case it will start just because of compression in the cylinders (the “back pressure” effect), in the second case the speed will also fall because the output power will be not enough.

Early era DC electric locomotives and early diesel-electric DC locomotives
Can’t say that the DC current and DC voltage can be directly compared with steam flow and steam pressure, but really the main common feature was the presence of back power in the moving and rotating drive. The early locomotives had conditional DC motors with heavy anchor or rotor (I knew one guy who succeeded to rotate a massive museum 1200 hp DC motor by one hand) that was put between the poles of powerful electromagnet and of course, the brushes and electric collector parts, now seen only in household equipment like electric screwdrivers or so. The main fundamental feature of this type of engine was big torque at low speeds and, of course, generation of “back” voltage, because this type of engine acted like an electric generator in the same time (the steam machines "in reverse" act as compressors). So bigger was the voltage and the load – the bigger was the resistance of the motor. In order to overcome this effect, the voltage of the independent winding (the traction DC engines have two types of them – the stator serial winding, connected to independent voltage source and the serial, connected directly to the anchor or rotor. The modern designs may have only the permanent magnet poles instead) was reduced that caused significant increase of RPM and current and of course, wear (when turned in without the load).
This type of speed control is very similar to cutoff in the steam engines, just because it reduces the negative force (DC back voltage in DC engine and counter pressure in steam locomotive), so it makes simpler to memorize.

I added (just for educational purposes) two videos, depicting close view of valve gear of Walchearts steam-distribution mechanism in two steam locomotives: one from the US and one from the Bulgary (EU), actually the Deutsche Reichsbahn Class 42, close relative of DB BR Class 52 you can drive ingame!

The first one represents the shunting on single steam locomotive and the second one normal movement from full stop to full speed. In order not to miss anything important when watching these videos keep in mind, that the "divider" (half-moon tilting bar) gives you vision of "virtual" percentage of cutoff that can be noticed by valve bar floating inside the divider. The lowest position ("fallen" bar) represents the max forward cutoff, the middle or near middle (near the axle of the "divider") the zero or almost zero, the max upper ("raised") - max backward cutoff position. The tone of exhaust depicts the approximate pressure and the throttle position (so FFF refers to low throttle and/or low speed, and SSH to nearly fully opened trottle or very high speed, sound between these two tones represent middle positions), the interval between these two sounds the speed and/or cutoff position. Sometimes, when the cutoff/throttle balance isn't met, you'll hear the "resistance" in the exhaust that may be noticed by loudness of exhaust, but in real life locomotives it is rare.

Lets watch closer to the first two videos.
On the beginning of the video you can't notice the bar, because it is raised in "backwards" position (like a "R" gear in the car). After a while they stop and change the cutoff to middle-forward position. Then, as you hear, the locomotive accelerates and stops. After that the driver pulls the cut-off to the middle (zero, neutral) position. This style of driving is very common when moving the underpowered consist, specially in scaled-down steam engines and not used when pulling too heavy consist.

The second video depicts low cutoff starting (nearly 40% or full or so. Note the wear on the "divider" on near middle cutoff positions) and then slowly changes ut to the near horisontal position (at the end of first minute of the video), so the amplitude of valve became innoticeable saying that the cutoff is set to near zero. Nearly after that you'll notice that the tone of exhaust dissapeared so you can hear clicking of gears, valves and bars. That's it. Now the locomotive is moving on near zero cutoff and near zero throttle. Near the end ot 3rd minute you'll notice significant increasing of cutoff (50% or so) from zero and the first part ends.
On the 3-4.34 you may note that the steam loco accelerates very fast from nearly 30% positions. And in the end of the fragment it uses the compression brake (you can note it leans slightly backward after full stop) with no live steam in (zero throttle or near it). The cutoff value is 50-60% of full down (Reverse) or so, taking in account that the driver missed with the direction of cutoff lever movement and then changed fast to reverse. After the full stop, the cutoff is set again to forward ~30% or so. At 6.40 you can hear clicking sound of steam air compressor.
Starting from the 10.00 you can notice the locomotive is now moving on live steam on low cutoff and not big throttle. This position is set up in order to go at 80 kmph or 50 mph constant speed, as noticed in the video.
On 10.45 you can hear very "tired" breathe, that seems to me, is caused by going uphill or simultaneous usage of the compression and train air brake.
Starting from the 17.04 you can notice the action of valve bar (if is falling down from the 50% reverse position to approx 30-40% forward). On the big values of cutoff the amplitude of the driving bar and gears is comparable, but on the small ones the differ very. If you watch the video in fullscreen 720p you can notice a little rusty cavern on the upper bar (the lowest bar, one of the driving ones follows the movement of the piston) that can help you to figure the amplitude and thus the cutoff approximate value.
On 18.57 you can notice shunting movements from the under cabin view but they can say you not a lot about cutoff and regulator positions (you can only notice the changes of steam exhaust only).

This walkthrough couldn't have ever written without the personal advices and help of LeadCatcher and Wolf. In the next parts of the manual, I hope, we'll see more good names in this chapter, so it can definetily help you not also to see how the stuff works but also to understand how to drive the locomotive by example.