Allow trains to send circuit signals indicating passengers

[nef]

The Shuttle Train mod is one which I like, but in (multiplayer) games without it, or for players desiring a slightly higher level of automation, I believe it would be helpful to make "passenger status" an available signal for circuit-connected trains, or else to specify a signal to be sent if there is a passenger in the train.

This would facilitate a circuit-connected vanilla shuttle train, and generally give players more options to use the train network as a means to move around larger factories.

[audigex]

Great idea - being able to summon a train by standing next to a gate, then have it leave the station when the gate sensor detects you've gone and the train sensor detects you're on board... perfect