Make sure you chat to everyone you find and explore every area of a level to get all passengers. They will disappear as you finish talking to them, indicating they have gone to the train.
Now all you have to do is keep them alive.
Medikits are VITAL for this achievement. Passengers can reach zero hunger levels, but not zero health as they instantly die.
1. Purchase MediKits at every opportunity. They are far more important than gun upgrades or ammo.
2. Try not to use MediKits on levels. Almost all the levels are straightforward enough to get through with a bit of thought. Utilise your melee and charge your melee for a one-hit kill on most shadow monster. If you end up taking a lot of damage, die and restart at your last checkpoint.
3. Try to wait for the exclamation mark warning to pop up to indicate a passenger is about to run out of health. MediKits fully restore health, so be sure to make the most of them.
4. Injured passengers have a Drip emblem indicating they are bleeding out - their health deteriorates much quicker, so keep an eye on them.
5. Be careful that you don't get distarcted by crafting or messages coming through on the computer. Prioritise exclamation marked passengers and train maintenance, then craft away.
6. If a passenger gets to zero hunger, their health deteriorates quicker.
7. As soon as you get the "YOUR TRAIN HAS ARRIVED" (or something like that) alert, hit the brakes and get out of there. Passengers will be fine and dandy while you explore the next level.
Tip for the Train Crafting Interface:
It's difficult to know that your cursor is over the "Craft" button when using the train computer, because the highlight colour is practically the same as the background colour.
In order to craft medikits whilst on the train:
1. Head to the far right of the train and press A once you're at the computer
2. Press down to hover over the crafting panel and press A
3. Move the cursor to the Medikit (box with a Plus sign) and press A
4. Move the 'cursor' over the X in the top right (which will close the screen), but instead of closing the screen press down once, you'll be hovering over the Craft button. Hit A as many times as you can to replenish your Medikits.