For some years now I have been considering adding sound chips (of the type used in greetings cards) for localised sounds on a layout (the layouts have come and gone, the job never got done). The chips typically hold up to 30 seconds worth of sounds and can be activated by a push button on the control panel or an LDR hidden in the track. On the railway itself, for UK outline, we had slamming carriage doors on cummuter trains, and a guard blowing his whistle, both of which would be nice added to the underside of a passenger platform. For US outline the guard shouting 'all aboard' could be added to a passenger station and the bells at a grade crossing. Off the line the sound of big sheets of metal being hit with big heavy hammers might be coming from an engineering works or the like.
This topic came up on a couple of other groups and some of the links might be of interest to anyone considering the idea . . .
Regards
Mike