It's quite likely that your car uses 2-6 volt bulbs wired in series. One socket is grounded with a wire going to the other socket. The other socket has a second wire going to the roller(s).
If someone placed a pair of 12 volt (14v ?) bulbs in there, they likely wouldn't light. Not that anyone would just assume that the trains run on that voltage, so the bulbs must be that voltage. And like Christmas lights, if one bulb burns out, the other one goes dark too.
When you unscrew those "vents" be careful not to scratch the roofs. The paint on the few I've opened seems to be pretty fragile.
I'm afraid I can't help you with the couplers as everything I know about them has been covered already.
Hope this helps,