Bret,
If you are switching cars with air, generally you want to work off the back of the engine, since that keeps the angle cocks that need to be open or closed on the engineer's side. On diesel-electric switchers, this isn't too bad if you sit ideways in the seat where you can lean out the window and still reach the controls.
On a gas/diesel mechanical, the controls are a little less friendly and far more labor intensive. By using a left hand drive, the engineer can be facing forward to watch for signals and operate the controls easier, and the trainmen don't have to cross over to open or close angle cocks even though you are switching off the front.
Maybe not the answer - just my guess.
Dan