As someone fascinated by cartography and transport, combining the two to provide something completely unique is an excellent step forward. I've always found it interesting to charter the Underground's actual routes through central London, rather than Harry Beck's iconic linear representations.
The map itself has a Google Maps foundation layer with the Tube lines layered above and yellow dots thereon represent the trains themselves. A video showing the site in action can be seen by clicking here.
This really is a fascinating site. We brought you something similar regarding shipping a couple of years ago that has proven popular. Here's one on aviation, too.
London Underground real-time train tracker
Shipping real-time tracker
Aviation real-time tracker