My crappy setup:

Going forwards:
Instruction:
I get in the first train (One on top), and drive forward, while holding left, or right.
Expected:
I go the direction I was holding.
Result:
I go the direction I was holding.
This works for all trains facing the direction you want to go, and including, and this is a key point, ones that AREN'T up front.
So, I can go in the second train, (The one on the left), and still go the direction that I was holding.
E.g. I hold left, I will go on the left track.
Going backwards:
Instruction:
I get in the first train (One on top), and drive backward, while holding left, or right.
Expected:
I back up slowly as though I had turned a wheel anti-clockwise, same as reversing in a car.
Result:
I back up slowly heading in the opposite direction I thought I'd go.
E.g. I'm in the top train, I press the back button to slowly reverse, holding down the left as though to turn left, I end up going the opposite way.
It gets even more annoying, when you add a backwards facing train, it fixes itself.
Like so:

So, if you're a terminus station, and you have a small little "get me to places" train of two locomotives facing the same way, and reversing, chances are you'll head down the wrong track.
EDITED: Added a my test save file.