The secret to winning missions with the least amount of resources spent are scouts.
Specifically, scouts of level 3 and higher,
for their sniping skill, killing one enemy instantly, for one rifle bullet,
and their camouflage skill, allowing them to become invisible anywhere after a short period of standing there.

Until you get to lvl 3 scouts, always try to bring scouts to points of interest and give medals to them to level up faster.

The great thing about sniping is that it's a one-shot kill from a far distance - so a single sniper can take out a whole patrol of 5 enemies by simply waiting, concealed, until the first enemy is in range (in silent mode so he doesn't start to shoot by himself), use the skill to shoot the enemy, and then simply run away while the rest of the enemy patrol scrambles and starts to aim at him.
Run away for a bit, maybe behind a corner, get invisible again, and rinse repeat.

If you need to avoid nearby alarms being rung, there is an even more overpowered, but more time-consuming way to deal with enemies:
using silent mode, walk into the sight cone of an enemy, so that the enemy gets a question mark above its head.
Then, move outside the sight cone.
This will make the enemy walk towards the sighted legionary for a bit (and then return to its post again) - but if you walk into the sight cone again, before the enemy turned back, the enemy will stop - and when you walk out again, the enemy will move a bit further towards you.
This way, you can kite almost every enemy out of a well-guarded place into any back-alley where this decoy himself, or any other legionary in hiding (for example a scout placed in the way using camouflage) can kill him using a knife.
With a bit of care it is even possible to lure away a single soldier out of a patrol, and deal with patrol soldiers one by one.

While playing on the highest difficulty, the order of the train cars never mattered. Feel free to do anything!

As for which train cars you need, the order of acquiring them is quite straight-forward:
locomotive,
storage,
3 infantry cars,
healing, (very important for both healing and crafting)
workshop, (very important for researching uniforms)
cooking (somewhat important for brewing coffee and transforming excess herbs into food)
artillery (rather nice to have to level up your workers)

As for which cars to upgrade:
First you should always upgrade your infantry cars to a level where you have comfortably more space than you need right now, for you might get new soldiers and it would be a shame to have to leave some behind..
Then,
upgrade at least one infantry car to the highest standard of cozy (and put your most demotivated soldiers there, while not all cars at as good),
upgrade your locomotive to drive faster,
and your healing car to allow research.

Next priority is to upgrade your infantry cars to level 1 insulation,
and your crafting cars to allow saving crafting materials (do that before starting to craft things).

Then upgrade your crafting cars to insulation level 1,
and your infantry cars to insulation level 2.

Then do whatever you feel like -
improving crafting proficiency is nice,
while higher capacity seems unnecessary (see storage exploit later in this guide),
as is higher hit points of cars,
or the rat-prevention (taking care of rats takes about 15 minutes with 4 workers? that's not a high price to pay..)

There is a great guide on how to level your soldiers in the first minutes of the game, before reaching Moscow, to save a maximum of people at the ... occurrence... there.

After that, your choices actually matter.

One thing to consider is how classes interact with each other:
As for combat roles:
Soldiers can have multiple classes and take some low level skills from one role while being assigned a different role for a mission;
For example, a soldier who is a medic and a grenadier can be a medic with the skill to throw grenades. (Also, you can use a dual-skilled for example grenadier+medic soldier as grenadier with the stabilize and medkit skills, to get the "win a mission with no medics" achievment)
On level 5, soldiers get the ultimate perks from a class, which they keep even when being assigned a different class.
So if your lvl 5 rifleman becomes a scout, the scout will keep the increased damage in melee,
or the lvl 5 scout becoming a rifleman can still run hidden as quickly as non-hidden!

As for stats, the most important is arguably fitness: It increases the amount of health your soldiers have, allowing to mitigate things going wrong (otherwise, your soldiers wouldn't get shot in the first place), and giving your grenadiers more range for their grenade throwing.
As for the other stats, to be honest, i never felt any difference if my medic had 7 int or 10,
or if my rifleman had 6 will or 10..

As for train roles:
Every role grants attribute points in two attributes (for example dex+char), which improve this role's efficiency as well as other train role's efficiency and even some skills.
So after you leveled up a role to 5, you might want to consider switching the soldier to a different role using one of those attributes (lvl5 worker gets a high amount of will, so switching to engineer gives the engineer a high bonus thanks to the high will, and switching to doctor afterwards gives a high bonus via int, ...)

Also don't forget to take into account the perks - there are some which are hard to ignore, like pacifist, which make it impossible to learn a combat role,
and others which can be ignored, but should not:
Thrill-seeker gives more combat exp, but decreases train-role-efficiency. So consider more combat roles for such a character (and while in the train, put him/her into the locomotive, where all perks seem to be ignored), or the artillery car, where efficiency does not matter.
Diligent on the other hand gives more efficiency, so put diligent characters more into the train - this does not give any benefit in combat missions!

Sometimes, you need the best of the best in a certain role (a hard mission might profit from a scout who knows what he's doing, or healing up soldiers quickly, you need that lvl 5 doctor), but usually, you can put a semi-competent soldier in charge, for the long-term benefit of getting multi-class professionals!

Do not underestimate the artillery car by the way, which gives your soldiers free experience in the worker role for no cost!
Also, crafting, research and even upgrading cars give considerable experience, so you might want to assign the soldiers who need the experience the most!

When you hover your mouse over an item in your crafting or inventory screen or when visiting a merchant,
you see how much space it needs, total first and for a single item in brackets,
and how much its worth, total first and a single item in brackets.

This lets you quickly see how much worth of resources you need to invest to craft an item.
In general, most crafting is good profit with materials you have found, and barely any profitable with materials you have bought, even if bough with discount and at a merchant with good prices.
So craft with what you have, not with what you buy, except if you have too much money or want the experience for your crafter!

As for where to find good prices - the stations directly on your tracks always have bad prices, while the ones you can send parties to (always bring someone who can haggle!) can have better prices.
The merchants with good prices offer good prices on everything by the way,
so even a merchant of flowers and cloth will pay well for medkits and bullets and weapons.
So once you found a good merchant, sell everything you don't need, until he runs out of money (maybe even buy all the resources he has that you might need).

When crafting, consider opportunity costs though:
If you have too many herbs, stimulants (which cost 20 herbs) are essentially free to build and give good profit: 200 gold worth of herbs give 247 gold worth of stimulant, for 3 hours of crafting
Once your storage of herbs run low, you might want to switch to medkits (5 cloth + 5 herb = 110 gold -> 134 gold, in 1 hour), but probably not before you have upgraded your infantry cars to insulation lvl2 and your other cars to insulation 1 and your uniforms to lvl 2.

As in real life, coffee is life, coffee is love, and it should never run out. +20% working efficiency for 12 hours, costing 5 herbs? That's essentially free, brew and drink coffee wherever possible.

My rule of thumb for points of interest is to investigate everything.
I have yet to come into a situation where i had to leave some behind because the reds were unto me...
It was only in the very last chapter where I did leave most PoIs uninvestigated - mostly not because of being forced to, though, but because I did not need any more exp or resources to comfortably reach Vladivostok.


The important thing is to have multiple parties out, ideally with the corresponding skills:
For the woods, get one with hunter, survivalist and herbalist,
for the lake, hunter and survivalist,
for the village, burglar and housekeeper,
for the merchant, the haggler,
for the lumber mill, whatever (maybe get the guy who regenerated energy faster).

Also don't forget you can send a group to a point of interest and then reroute it to a different point after it is done with the first! (But probably not after chopping wood, for this is very energy-consuming)

Investigating points of interest is also a great way to skill up low level roles (so a soldier who is a level 1 or 2 scout is pretty useless for example, let him investigate as a scout to get experience there, before bringing him as dead weight in a mission!)

In an ideal world, you could give every person a job and be done with it, but sadly, we're in (almost) Soviet Russia, so jobs come up and demand people..

Thus, while it's more convenient to keep three to four squads (one for villages, one for woods/lakes, one for merchants and one for lumber mills), you probably will need to assign people from there to other tasks while not on missions.

This also leads to a weird mechanic which can be exploited: When legionaries are in a squad, their equipment does not count towards your storage -
So if you have too many things (as in the screenshot here)

you can just put some people into squads, they will take weapons and ammunition and consumables and thus empty your storage car, probably enough to drive to the next merchant.