Auto-fueling for personal trains (like cars!)

[wspikes]

In 2.0, Cars gained a feature that automatically fuels them when you get in and start moving. This has been quite nice to have!

Some players like to use a locomotive as they would a personal car. It'd be nice if it were possible to auto-fuel the locomotive as well.

Suggested activation conditions:

1. The player is physically in the train (no remote view)
2. The locomotive has no fuel
3. The player does something to start the train moving
   1. Forward/Backward Movement Hotkey
   2. Train schedule + go (including temporary stops)

Fuel-wise, I think it'd be safe to use the same basic code as cars, tweaked to use the correct fuel category for the particular vehicle.