So I had an epitome from an XPtoLevel3 video to run a train heist campaign, where the players are the ones robbing the train. Does anyone have advice for how to run this, aside from the obvious stuff like alignment options.

Well, the first thing to consider is how long this will be, and how much of it is going to involve the actual train robbery. Will you run it over 5 levels, with 2 levels being preparation for the robbery, one level being the robbery itself, then 2 more being the getaway? That could be a good way to go, as it means the earlier levels will be more heavily roleplay and the later levels more heavily fighting, so they are not fighting whilst they are squishiest.

The important elements of a heist game are the twists and planning, so for you to plan a heist for the players you will instead need to plan a detailed and regimented system of defence, with strict rules to follow, so that they can plan around it through espionage and such. Don't try to plan for what they will do, but do consider how they will go about gaining information so that they can make their own plan.

For example, some key things which they would benefit from would be:

• Blueprints of the train
• map of the train (these are different - blueprints show access hatches, ducts, and all that. Map will show only carriages)
• Map of the route, for perhaps a quarter mile either side of the tracks
• roster of employees (stokers, driver, engineers)
• roster of guards (from a different source)
• Guard routines (from observation, or becoming employed as a guard)
• blueprints or information on the defences (locks made by who, what spells are in place, what to expect)

Other things to consider which are important would be

• a twist to the routine, which they need to adapt to on the fly or find out about late in the plan - a visiting royal using the train as transport, for example, would increase the guard.
• henchmen - make a list of useful NPC's which the party can use for the boring jobs, like keeping the horses together for the getaway
• a non-combatant NPC who can guide them in planning by asking questions. A child would work well - someone curious about everything. They might be the ones who ask "how are you keeping the horses beside the train whilst you're in?". They could also be a contact for the NPC henchmen, saying "my mum knows a woman who can do that, she's the best at horses" or something like that.

You will need to become intimate with the layout and routine of the train yourself to make it work well. You need to know how long it takes to get to each point on the map, how long the party might have before a patrol notices a dead body, that sort of thing. To make it work best, it needs to be tense - you can't have it escalate into "let's just kill everyone on the train" and consider it a successful heist!

Also consider the obstacles you put in place - if possible request the players give you characters well before time so that you can plan to make each of them vital to the heist - heavy doors to lift, narrow walkways to block, magic locks to dispel, normal locks to pick, stealth sections to get keys or codes, that sort of thing. Really lean it into every character having a niche - though they may not use themselves how you planned, the option being there will make it feel really good if they do use it! It's going to feel bad if there's a magical barrier and nobody chose spellcasting, whereas if you know they didn't you can have them work out a way around it (perhaps onto the roof and down behind the barrier).

Most importantly, be flexible (ok, most most importantly is have fun, but second comes flexibility). Don't plan solutions because the players will almost certainly make their own. Allow anything that could work to proceed, and don't get caught up on anything you planned to happen. Perhaps their first move is to cast sleep on the guards and then dehitch the guard carriage, which may muck up your whole expectations, but will be an awesome plot point and one they will remember for years, provided you let it happen!

Thank you so much! I had some vague plans, and I really like your ideas for a campaign layout, so thank you very much!

You Son of a *****, I'm In!

That said, Thoruk gave some really good advice. These kinds of games usually benefit from quite a bit of "yes, and" to make them enjoyable. That is, don't be afraid to improvise and change things if your players come up with cool solutions to problems you hadn't even thought of. At the same time, you might want to help them stay on course a bit if they stray too far. Best of luck and let us know how it went! :D

Thanks mate!

If you want to run it in an established setting, Eberron is made for stuff like this. They even have a "Lightning rail" that runs all over the continent. 
There will be plenty of existing maps, mechanics, lore, etc to steal/pull inspiration from so you can have a fleshed out world and be able to concentrate on your story.

You see that? Tell me your party wouldn't want to recruit that halfling tetradactyl rider for their heist.

Thanks, and you have a very, very good point.