London Transport

There has been some discussion on other forums concerning the arrangement of third rail power systems. Several years ago, while riding the London Underground (can't remember which line for sure, but it was likely the Central Line) I noticed that there were not three but four rails. The third rail was outside the main rails as expected, but the fourth ran between the main rails and appeared to be worn on top from a shoe.

Does anyone know how the power system is set up on the London Tube? Was this indeed a power rail?

I was just over there two weeks ago and had the same question. Luckily there was another railfan in the group who said that the fourth rail served as a ground.

Thanks. About 5 minutes after I wrote the question, I found it on a UK railfan site. They do indeed return the circuit through a dedicated fourth rail, insulating the drive wheels from the electrical circuit. I wonder why they made that decision? Increases the amount of steel needed by a third.

NHRR WTBY wrote:Thanks. About 5 minutes after I wrote the question, I found it on a UK railfan site. They do indeed return the circuit through a dedicated fourth rail, insulating the drive wheels from the electrical circuit. I wonder why they made that decision? Increases the amount of steel needed by a third.

The original reason was the danger of damp in the tunnels. The London Underground was earlier than most other metros and modern technology would probably make the fourth rail unnecessary. However changing the whole system to third rail only would be very expensive.

Some formerly fourth rail lines have been converted to third rail only. These were the lines built by the LMS (London Midland and Scottish railway). Those parts where London Transport trains no longer run have had the fourth rail removed. Clearly third rail only trains can run on the fourth rail sections of these lines (mixed Bakerloo line and Silverlink Metro).