Coach Lighting The Easy Way...

As it happens I did a bit of mooching around Amazon last night, found some sellers with normal magnetic reed switches and magnets.  Thinking about it if you bust out the switch from those sets of lights you're getting and wired the two poles of the reed switch in place of the switch all you'd need to do is figure out a place to put the magnet where it keeps the reed switch closed. 

A typical reed switch is 'open' (i.e. off), when you put a magnet near it the switch closes (i.e. on), although you can get some who work in the opposite manner.  I should imagine the way the hornby board works is to use the activation of the reed to turn an electronic switch on or off each time the magnet activates it.  As we don't have that board we'd need to keep the switch in a closed postition all the time you want the lights on.  The way I'd approach it would be to fit a small piece of metal inside the body of the coach along with the reed switch and then you could probably place a suitable small magnet on the outside of the coach.  That should then stay in place due to the attraction to the metal plate and in turn keep the reed turned on while its in place.  Sensibly you'd put the reed on the bottom of the inside of the coach and stick the magnet underneath the coach.  If you get some of those powerful little Neodymium magnets they should be more than up to the job.

As for a light going on when a train approaches, you could if you got a two pole reed switch have it display say a red LED by default when there is no magnet near it and then when a magnet comes close switch it to the other pole and turn off the red LED and turn on a green LED for example (think here stop lights on a layout).