[zaroth]

You don't even need GPS, or an external signal at all. All you need is an odometer.

1) Engineer selects the train's starting position (the station and departure gate) and selects the route.

2) Train software presses the equivalent of a 'trip reset' button that our cars have had forever. Current Position = 0.0

3) The train and the engineer each program a maximum speed. The train determines the maximum speed by selecting the maxSpeed which has the highest Position less than currentPosition. The engineer determines the maximum speed as she normally would.

4) The train's speed must not exceed the lower of the two configured maximum speeds.

[tuna-piano]

And you don’t even need mechanical connection. Just a simple over speed alarm would seem to help.