London Underground tidbits

I’ve long been fascinated by urban subway/metro/underground rail systems (in my youth, I was a big fan of the Paris Metro) and lately, I’ve been watching a couple of British Rail DVDs - one of them is focused on the London Underground Metropolitan line. Here’s a couple of interesting bits of information, that I learned while watching this DVD and from subsequent reading on related web sites:

  • The London Underground network reaches out into some quite rural country - one example of this is the Metropolitan line (which incidentally was originally built as a mainline railroad), which even has a single track branch. In the video, you can see the train threading through heavily wooded areas on the Chesham branch … Parts of the Metropolitan line were steam powered until the second half of the 20th century and reached even further out into rural country.
  • The London Underground is electrified using a 4-rail DC system. This is quite rare and differs from the more common 3rd rail system by having a fourth center rail between the two running rails. The voltage used is 630 V DC, with the outside (3rd) rail at +420 V and the middle (4th) rail at -210 V. In some cases, where tracks are shared with regular mainline trains that use rolling stock equipped for the more conventional 3rd rail system, the outside rail will be at +630 V and the middle rail at 0 V - the difference in potential between the two rails is the same either way and the trains will work just fine.

Another peculiarity of London Underground and suburban railroading is that in some locations, you can find tracks electrified using the 3- and/or 4-rail DC system running next to tracks electrified with overhead catenary at 25 kV AC. One example of this is on the tracks out of Euston station on the West Coast Main Line.