Coach Lighting The Easy Way...

There's a little black on/off switch on the battery pack, next to where the wire comes out.

I meant more to turn them on and off without having to open up the carriage or having the switch/battery on the outside.  I suppose you could drill a small hole in the bottom of the carriage so you could get access to the switch with a pen tip or something...  Worth investigating as a cheaper option.  I'll have to look into the magnetic reed switches, I bet some enterprising little man from the far east probably sells those little control boards that come with the Hornby lights