I'm not sure when, if ever, I'm going to complete Train, so I'll just share my findings now.
For the first car when going for box kills, after you remove the BA from a guard, he may be outside the explosion range from the box. At the small cost of one bullet, firing by them to alert them and put them into their ready stance moves them forward enough to guarantee a box kill, so long as you're hitting far enough into the box.
For the front of the second car, by doing crouching quick leans, you can get the guards to blow up several of the boxes, often box killing the left unarmored guard, and on rare occasions killing the unarmored guard on the right too. You can also slowly drag the BA guards to the front with enough quick leans, and even get them in between the doors, allowing for easier kills. Slapping them is also possible, but somewhat dangerous as they can't be stunned until their BA is removed. This strategy is quite a bit slower than 1.1, but it's somewhat more reliable, but also very stressful to do.
At the back of the second car, if you drag the one BA guard to the silver crates, you can move around them in crouch to also drag him into the corner completely safely. Also, while it's possible to run behind him to do the frozen guard glitch, it's actually possible to crouch at the far left, and run in crouch to the side of the boxes to freeze him. This allows you to freeze the guard with no danger of getting hit. The only issue is this trick only works on NTSC, and not on PAL or JAP.
All of the unarmored guards that run freely around in the third and fourth cars have some interesting AI. They get lost very easily and will at random times continue to run towards you. When they reach you and they're still in a running animation, they won't stop until they're done, take longer still to react, and then finally fire at you, making them very easy guards. The other thing is that every time they see you (via quick leans, etc), they will take a long moment to react, then run a few steps towards you and stop again. You can use this to zone them well, and to safely control and drag them to the glass rooms in the cars.
There are two guards in the third car that are less likely to approach and will generally stay in place, but the front one can be lured to the silver crates like the other five regular guards, and the back one can just be slapped after forcing a firing animation, or waiting for them to crouch. He always starts far back in the third car, but will almost always do a running animation when he first sees you, bringing him to a safe distance to run and slap him.
The four black BA guards in the fifth car that can be randomly placed can be AI abused too. They normally stay in place and rarely leave their rooms. The reason for this is while they can randomly run around, they can't seem to open doors. So if you open doors while backing away and taking cover, you can slowly drag them to the fourth car where all the glass rooms are to take them out safely. They can even be slapped if you're willing to be risky to conserve ammo.
One other small thing that is common knowledge, but not used in any of the three completions is you can be seen by Trevelyan without activating the rest of the scripts, so long as you don't enter the room and trigger "That's close enough!". This makes shooting the locks from the outside much easier, and you can easily leave as few as seven left when you enter. The minimum is six, though the last lock is difficult to get from the outside, and probably not worth trying for.
To be honest, I'm somewhat of a weak player when I don't have consistent strats and can't flow chart my way through a level. I really need a better strategy for the front of the second car before I'm willing to try some more. I've already pumped 20+ hours in and have gotten nowhere, lol.
