Hi Greg,
On the rear of many LGB loco's are sockets, ostensibly, to supply power for coach-lighting..
If you remove the track connections on a loco, it can also be used as a 'fast and dirty' way of feeding battery-power into a loco for test purposes..
Indeed, many people don't just use it for testing, but as a permanent way of feeding power to the loco from a trailing battery car. It's not the best method, but it DOES work and is easy to do.
It should always be remembered that unless the track pickups are also removed or disconnected in some way, as Phil says, then when you're feeding power in via the socket you will actually be feeding the battery power to the whole track!
The plug from the decoder board to the rear socket is usually light blue in colour, unlike the rest of the CT plugs which are usually white. No idea why this is, but it's a useful identifier.
Jon.
