Third Rail Type

[tube10]

For the london underground is the third and fourth power rail of the top,middle or bottom cntacttype?

[Deleted]

Top running, sadly.

[Deleted]

It was a good design decision at the time. To obtain the desired number of trains, automatic signalling was necessary, operated by track circuits. With the technology of a century ago, this was much easier to achieve if the running rails were kept free of traction currents, so two conductors were needed. Overhead was obviously impossible in the tubes, so a four-rail system was adopted.

Side contact would have required higher conductor rails, so that the collector shoes would clear the running rails, and there would not be sufficient clearance for these under a tube train.

[Deleted]

Yes, it made sense at the time. They'd never do it like this nowadays though, it's nearly all bottom running now, like the DLR for instance. They'd never make the tunnels (and therefore the trains) so small again so space wouldn't be an issue.

[Deleted]

The larger the tunnels, the more the taxpayers will have to provide to meet the losses. Even at their present size, the tubes have never made a worthwhile profit.

Even with the present size tunnels, there is now no problem in implementing track circuits (or other safety technology) on a three-rail system, and I suspect that there would be enough clearance in the tunnels for a side-contact or bottom-contact outside third rail.

[Ben]

It does have something to do with Earth leakage though, and electrolysis. Yerkeys was a shrewd man. He didnt want even the possibility of the LCC taking him to court for possible damage to their sewers and pipes. However, 2 power rails makes possible the future use of 2 or 3 phase AC with all those benefits. This, though, is immensley unlikely ever.