N Scale: Infrared Control
The other control system used for indoor flight is infrared. Similar to televison remote controls, this technology is well suited to indoor environment and the short distances involved. The monorail would need a receiver and motor control but all this was easily small enough to fit.
Didel and others have some IR transmitters and tiny IR receivers. The key is to make sure the receiver is always aimed at the transmitter, difficult when the monorail goes through a building. Repeater stations for the transmitter may be necessary. Still, this technology looks promising and may be more resistant to interference than radio control.