Far From Home
Finished Chapter 1

Complete Chapter 1. You can have a total 18 Legionaries on your train. Some of it subject to either RNG, or restrictions on the first playthrough, and you may have less due to it being your first playthrough. you can take a full squad of 10 into the battle of Moscow or only 8. You may need to bring only 8, to unlock some hidden legionaries that will not appear unless you bring 8. ShepardofCats made a guide for powerleveling if you want to do that to maximise XP, roles, and keeping 2 extra legionaries from your initial group.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3113188882

To Go West, Go East
Finished Chapter 2

4 Legionaries will join you at Penza Station, culminating in a choice to go North or South, with their associated achievements.

Gathering the Legion
Finished Chapter 3

Going north will start with 1 legionary, to a total of 16. Assuming you have completed side missions looking for legionaries and always taken the option to keep them on the train.
Going South will usually net you 6 less. RNG may be at play to net you 4 less only, as other people claim, but I only played through this game twice, and the first playthrough condition may apply.

Safe Passage
Finished Chapter 4

You can change your train car in the beginning of this chapter.

A Golden Opportunity
Finished Chapter 5

A timed chapter. You've got a train car carrying gold to catch.

Children of the Storm
Finished Chapter 6

During an ambush on the tracks in a mission called "Canyon, 4 Legionaries can be found during that mission. 4 more will join you in a few stations down the track.

The Cold Within
Finished Chapter 7

1 Legionary, and it was Dorota Vlckova both times, will appear at the station for you to pickup

The Decision
Finished Chapter 8

Another choice to go North or South. Both routes don't make much of a difference. You still have to deal with blockades and missions, but with no legionaries to pick up either way.

The End of the Line
Finished Chapter 9

Like all achievements before it, it cannot be missed unless you just quit playing.

Going North
Chose the north path when leaving Penza

There are a maximum of 47 legionaries that you can pick up with this path. You can save/load for the other Going South achievement, but you will still need at least 2 playthroughs to 100% this game.

Going South
Chose the south path when leaving Penza

There are a maximum of 41 legionaries that you can pick up with this path.

Forager
Gathered extra resources while foraging

The noteworthy traits that your legionaries can have for getting extra resources on foraging deployments are:
Herbalist, Naturalist, Hunter, Housekeeper, and Burglar

I Also Like to Live Dangerously
Went on a mission without a medic

Not difficult, Save, Start a deployment mission without a medic, and reload after the achievement pops.

The Real MVP
Gave 10 medals to a single soldier

Taking your time, send the same 3 soldiers on mission deployment, and keep giving them medals. So 10 missions. No need to stress over this one, as even when rushing, if you take care of a dedicated combat squad's stamina, you will always be able to send them on the many missions that are available. This also requires that you complete the objectives of the missions rather than retreat.

We Are Many
Successfully completed a mission with a full squad

There's a maximum of 10 soldiers to make a full squad.

Strategic Retreat
Left mission without completing any objectives

Save, start a side mission, and retreat. Not applicable to the story missions that require mission completion.

They Had a Blast!
Killed 7 enemies with a single grenade

Keep in mind that achievements in the main game cannot be achieved in DLC scenarios.(I tried) There are a multitude of opportunities involving attack, and stealth missions. Missions that put you on the defensive may make it a little tricky. Taking your time and finding the right place to do it, when finding a big patrol, or throwing a grenade because you're overwhelmed by a dozen people shooting at you are two different things.

Yoink
Plundered the village storage during Help is on the way quest

Prior to Penza station, usually between Ryazan and Ryazhsk Station. The quest "Help is on the way" will appear, along with a new location for a combat squad to deploy to called Chernobaevo. No prizes for guessing where nuclear power plants will be built in the future.

First Silent Blood
Successfully performed stealth kill for the first time

This can be done in the tutorial mission "A fateful Encounter" in Sukhinichi. And every mission after it.

Ride of The Valkyries
Successfully completed a mission with an all-female squad

You will have at least 3 women available after escaping Moscow. You can try it immediately, or wait until you have more female legionaries. Keep in mind that you have to complete the mission entirely.

Eat Lead
Shot over 500 bullets during one mission

If you feel like wasting ammunition, the Machinegunner class is great for this. As long as you find an enemy soldier in very good cover, and you find some good cover to yourself, both sides can expend their ammo until you get the achievement, and reload the save, disgusted with the waste of ammunition. You could also use the suppressing fire ability to fire at nothing until you're dry, assuming you have several machinegunners with over 500 rounds to spare. Might even get this achievement naturally if you go out with a balanced squad and aren't trying to minmax ammunition.

Tanks a Lot
Boarded a tank during a mission

A tank, isn't an armoured car. Earliest opportunity is the Mission at the end of chapter 4, Omsk. You'll have to shoot soldiers next to it or stealthily kill them to prevent them from boarding it first of course.

Look at Those Fireworks
Used the Artillery Strike 8 times during a mission

Requirement: 2 artillery cars, manned cannons, and the required shells of course. Might need upgrades as well.

There's no achievement for the upgrading and insulating the artillery Car.

In a Clover
Fully upgraded an Infantry Car

Total Required Materials:
-Wood: 400
-Metal: 250
-Cloth: 800

This Bad Boy Can Fit So Much
Fully upgraded a Storage Car

Total Required Materials:
-Wood: 350
-Metal: 285
-Cloth: 0

Fast and Furious
Fully upgraded your Locomotive

Additionally, keep in mind that any upgrade to the locomotive car voids the "How Did We Get Here?" achievement.
Total Required Materials:
-Wood: 200
-Metal: 985
-Cloth: 300

Michelin
Fully upgraded a Kitchen Car

Total Required Materials:
-Wood: 800
-Metal: 305
-Cloth: 300

DIY
Fully upgraded a Workshop Car

Total Required Materials:
-Wood: 600
-Metal: 375
-Cloth: 300

No More Boo-boos
Fully upgraded a Hospital car

Total Required Materials:
-Wood: 1050
-Metal: 350
-Cloth: 650

Go-getter
A soldier reached the rank of Second Lieutenant

Soldiers gain XP from being on duty. Mission Deployments XP and awarding medals give a big lump sum of XP. Deployments to merchants do not give XP.

DaVinci
A soldier acquired six roles

Most starting soldiers who already have a combat and train role, while also starting at the rank of private are capable of this, as later in the game, new soldiers you pick up start at a higher rank, and miss out on role points.

Igor Pachmelnik Zakuskov
A soldier reached level 5 of the Engineer Role

Leveling this role can be done on the Locomotive, and the Workshop Car

Proletarian
A soldier reached level 5 of the Worker Role

Leveling this role can be done on the Locomotive, all Car upgrades and repairs, and the Artillery car.

Babica
A soldier reached level 5 of the Cook Role

Leveling this role can only be done on the Kitchen Car

Doctor Aybolit
A soldier reached level 5 of the Doctor Role

Leveling this role can be only done on the Hospital Car. The research and crafting occupation will help for XP when no soldiers are injured.

Demolition Man
A soldier reached level 5 of the Grenadier Role

All combat roles can acquire XP through foraging or mission deployments. A class you will need for the "They Had a Blast!" achievement, and to deal with tanks/massed infantry.

Health Charger
A soldier reached level 5 of the Medic Role

All combat roles can acquire XP through foraging or mission deployments. A class that most people have for their skills, which they use to multiclass.

Son of a Gun
A soldier reached level 5 of the Rifleman Role

All combat roles can acquire XP through foraging or mission deployments. A class that stands out for its bayonet charge skill.

The Heavy
A soldier reached level 5 of the Machine Gunner Role

All combat roles can acquire XP through foraging or mission deployments.
The least used class by minmaxers, but it's nice to spray bullets at communists charging at you in defensive missions.

Mordecai
A soldier reached level 5 of the Scout Role

All combat roles can acquire XP through foraging or mission deployments.
This is the most used class due to its ammo conservation, skills.

I am Speed
Made it to Vladivostok in less than 20 days

As long as you never stop to forage or detour, you can easily get there on the grandpa engine in 13 days without even hitting overdrive or upgrading the Locomotive, and only repairing when you're clearing a blockade.

How Did We Get Here?
Got to Vladivostok without upgrading the Locomotive

Might go about doing this with the "I am Speed" achievement. You can upgrade any car as long as it's not the Locomotive car. You can even switch out to a different Locomotive. As long as you do not upgrade it, it will count.

Historically Accurate
Except for soldiers in the Moscow trainwreck, nobody died during the journey

Do not let any soldier step off the train in any scripted event, and stay for any reason if you want to be on the safe side. This includes transporting the family, send them directly to their destination despite Captain Langer's Misgivings. Check the Legionary checklist to keep count for maximising your legionaries.

Druzhina
Finished the game without losing anyone from the original squad

Keeping your Legionaries alive isn't the most difficult, as long as they have their morale, and aren't exhausted at the same time, it's unlikely you'll find them sticking their head in an oven or firebox. Save scumming missions is an option if you keep losing them to tanks.

Slow and Steady
Didn’t use Overdrive train speed during the whole journey

Keep in mind that ramming any obstacle will result in forcing a train into overdrive. easily done in combination with any achievement.

Finally Some Good Fricking Food
Cooked Goulash 10 times

The Kitchen car and researching the Goulash recipe is required for this.
10 food and 50 herbs are needed for the recipe.
30 food and 110 herbs are needed for the Goulash itself.

The Journey Ends Here
Ran out of fuel

Another easy one:
Save your game at a station.
Sell off almost all your coal at a station.
Run your locomotive and reload when the achievement pops.

History Lesson
Read an encyclopedia entry

Only difficult if you happen to be illiterate. In which case, I'm impressed that you made it to this guide.

Succumbed to Al Ghul
A soldier became an alcoholic

You have 2 options to run through this achievement:
a lot of legionaries not feeling great, usually an event causes that. Save and feed them all the Vodka you stockpiled. Either that or stop at a station with a pub to buy them drinks. Reload and get them drunk until the achievement pops.

The More The Merrier
Picked up 15 Legionaries on your journey

Almost all legionaries are picked up in stations, or side missions regarding specifically searching for them.

Nobody Left Behind
Pick up all Legionaries throughout multiple playthroughs

Can easily be done on 2 playthroughs. Easily missed legionaries are because you managed to build a squad of 10 legionaries in the Moscow battle, and the Moscow legionaries were not available to you. Or you cut down a hanged legionary in a side North Route exclusive mission, in Armyzaz which scripts 2 of 6 legionaries to die. Check the Legionary checklist to keep count for maximising your legionaries.

Choo-Choo
Found the choo-choo button

Zoom into the Locomotive carriage and press enter

Coal Baron
Gather over 1000 Fuel

Easy if you're taking your time. Grandpa train has the least fuel consumption. Deployment missions are a great source of Coal, and general resources, so never pass them up.

Why Is Everyone Coughing?
Kept the soldier with tuberculosis on board of the train

Around chapter 6-9, assuming you sent your soldiers out foraging, one of them will come back spitting blood, as a symptom of tuberculosis.

Generally unneccesary for me to do this as Eneldo has made a detailed guide for everything you'd need to do to complete certain objectives:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3242779204

The Incident Story
Freed the Captured Soldiers

Complete The Incident Story

The Machine Gunner Story
Helped Seize Chita

Complete The Machine Gunner Story

The Cat Story
Rescued the Kitten

Complete The Cat Story

The Long Story
Destroyed the Tanks

Complete The Long Story

The Cheka Story
Defeated the Cheka

Complete The Cheka Story

The Tank Story
Got Out in Style

Complete The Tank Story

The Factory Story
Saved the Civilians

Complete The Factory Story

The Resistance Story
Completed the Last Stand

Complete The Resistance Story

The Baikal Story
Fought at Baikal

Complete The Baikal Story

The Blockade Story
Left No Car Behind

Complete The Blockade Story

Ready for Anything
Unlocked All Squads

You will need to unlock every squad available for every chapter. Start with whichever squad makes that chapter or uncompleted challenges easier to complete.

Work Hard, Play Hard
Completed All Mission Challenges

Consider them optional or secondary objectives in general.

Make It Harder!
Completed All Optional Challenges

Pacifist challenges are horrible on their own, and something to consider. Obviously you can't pair them with massacre or challenges that involve killing.

Challenger
Completed All Challenges

Keep in mind that 100% means completing every mission 4 times, each time with different squads, but it is not necessary to complete every challenge at the simultaneously, so spread these challenges out between each run for an easier time.

Commander
You Did It!

Finish every chapter with every squad, and complete all challenges

You only need 2 playthroughs for all achievements.
Take your time on the first, and rush the rest of the achievements on the second.
Take North of Penza if you like hanging out and chugging along slowly with more Legionaries and South of Penza if you feel like going through faster.

Here's a quick reference for the number of Legionaries you should have when you start the chapter at the corresponding station. The only big influence on the routes for the numbers of legionaries you can gain is at Penza Station. The second decision of North or South is generally trivial.

Legionary Max Count Northern Route:
Chapter1(Moscow): 18
Chapter2(Penza): 22
Chapter3(Kazan)(North): 23
Chapter4(Yekaterinburg)(North): 38
Chapter5(Omsk): 38
Chapter6(Novo-NikoLaevsk): 38
Chapter7(Nizhhneudinsk): 46
Chapter8(Chita-Bockarevka)(North): 47
Chapter9(Bockarevka)(North): 47

Legionary Max Count Southern Route:
Chapter1(Moscow): 18
Chapter2(Penza): 22
Chapter3(Samara)(South): 23
Chapter4(Chelyabinsk)(South): 32
Chapter5(Omsk): 32
Chapter6(Novo-NikoLaevsk): 32
Chapter7(Nizhhneudinsk): 40
Chapter8(Chita-Harbin)(South): 41
Chapter9(Harbin)(South): 41

You will always need 4 Infantry cars for picking up every legionary, but only in the South route will you not have to upgrade one car to Max capacity.

ItalienSturm offers a more detailed breakdown on legionaries:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3149072182