The video below is a good starting point, but then read on for a description of how TCP/IP is actually transported over a Lego train carrier. Basically, on the back of each train is a USB memory key (literally a key in this case; the French have a good sense of humor) with four dangling wires attached to each of the key’s interface pins. The train chugs around the track until it reaches a station (a relay turns the power off when a train approaches), where an Arduino board interfaces with the USB key via a very complex piece of cardboard wrapped in aluminium foil (see gallery below).
This is the most utterly useless thing I've ever seen, but it is so amazingly cool that I'm envious of the creativity.
