+ rail

The positive rail is +420V and the negative rail is -210V because of a combination of historical design and materials science.

Back when the electrification system was first designed for the CCE&HR, GNP&BR and the BS&WR, the materials science needed to properly insulate a 3rd-rail +630V electrification delivery system was not entirely well-formed. There was substantial concern that galvanic corrosion and leakage to earth would result, due to the lesser quality of material used to insulate the electrical cabling, and that the cast-iron tunnel rings, as well as other metallic service conduits, would deteriorate as a result. Therefore, the system was designed to use two power rails, with the outer one held at +420V and the inner one held at -210V, by using a pair of resistors connected to earth in such a way that the two rails 'floated' at the appropriate voltages under normal conditions, but would 'fail safe' to +630V on the outer rail for a short period if an earth fault took place, preventing total system failure and allowing the fault to be more easily detected.

The same materials science also contributes to why the outer rail is higher than the inner rail - the closest part of the train to the outer rail is the positive shoebeam, which is nowadays made of fibreglass, and may have once been made of wood. The inner rail, OTOH, is close to most of the train's metallic body, as well as the underfloor equipment and the bogies, and thus needs to be lower to the track.

fx: stampede of T/Ops and T/Os and infraco personnel...

I have a sneaking suspicion that the information thus far is still only part of the story.

Unfortunately, the relevant textbooks are an entire country away; but I feel that there was a precedent set by the CSLR which was third-rail and echoed by the CLR (which was also third-rail) that the posi rail was XX inches higher than the running rail - that relationship has been perpetuated since the change from inner third, via outer third and fourth to the current (badoom, tish) set up.

There is no real technical reason why the posi rail is higher than the negative, it all depends on how accurately you can keep the track geometry when you get to points and crossings - if a clearance of 2mm could be maintained between the top of the running rail and the bottom of the collector shoe, then I'm sure the posi wouldn't be as high as it is now, if it were not for historical reasons.

The height of the neggy is a much more complex issue - possibly as involved as why Brunel didn't choose to have 7' instead of 7' AND ¼" - I suspect that the neggy was originally sited so that is fell into a statistically suitable zone, where it was higher than any possible shoe drop onto the running rail and lower than any coupling link (remember those....) could fall and make contact. That is not to say that the solution adopted is perfect, a friend of mine can remember a condensing pannier running along the District and the front vacuum bag running along the neggy gently sparking along.

A quick look in 'Tube Trains Under London' reveals that the the C&SL used a centre third rail below running rail level, and the W&C one at running rail level - in the both cases with fairly complicated arrangements (lots of wooden ramps, etc) to get the collector shoes over the running rail at pointwork.

A few years later on the CLR the (centre) third rail was just raised a bit (by how much? - J Graeme Bruce doesn't say) so the shoes just sailed over the running rails with no problem.

Both the GN&C and Met/MDR Kensington experiment used outside third and fourth rails - in both cases raised, but to different specifications.

The standard system (that we have now) was first used in London on the Ealing & South Harrow - electrification work started 1902 - it went on to be used not only on MDR & Met, but also the Yerkes tubes (later LER).

I would assume that once the decision on slightly raised outside third centre fourth rails positioning was largely a matter of defining a standard that wouldn't cause too many problems with bits of trains hitting the rails - given that it would be part of the national system [1] - and shoes hitting things on the track.

But the Mersey Railway - electrification started 1901 - used the same standard - AFAIK, I've not seen any reference to its conductor rails having moved (which I have done for the Tyneside & LOR, both of which were brought into line with the standard), as did L&Y.

So did one adopt the other's standard (and if so which way was it?), or was this chance, or down to having the same engineer? Or had it come over from the USA?

[1] BR had restrictions on what could work over SR lines for this very reason.

I think you'l find - sorry, I wasn't around at the time(!) - that the conductor rail positions on the underground predate main line electrification. I don't know, but it's entirely possible that the main line copied the Underground and the Underground copied the USA. At the time of District and Met electricification in the early years of the 20th century, the Americans has a lot of influence.

Mounting the positive rail in the tube any lower than it is would be hard. Central and Northern are particularly tight. On some sections, the rail can get higher than normal. Central was famous for requiring high lift shoe-gear. Also, in places, all that will fit is an "L" shaped angle iron section but mounted upside down. If the rail was any lower then a lot more of this sort of compromise would have to be used. As mrfs42 says, the standard is what was decided at the time for what at the time were good reasons. Those reasons won't have changed as they were largely to do with geometry, that that's where it stayed.

Negative rail is lower as there isn't the same space contraint on the track, and as much space as can be found is required for the underframe equipment.

I'd like to revive this thread, if I may?

I would love to know why the 420/210 and not 315/315 or 483.7/146.3 or any other random split!

I'm also fully aware that the neg is 1.5" above running rail height and the pos is 3" - double the height *and* double the voltage. But I still don't get why!

Please explain in words of 2 syllables or less. I'm a bit fick, me.



PS Thanks to all previous contributors to the thread - I've learnt a lot already

I wouldn't be surprised if the 210/420 volt split is because the latter is double the former. It could be coincidence, but other standard voltages seem to exhibit this pattern as well - the 1500 volts used by the T&W Metro is double the 750 volts used by trams and mainline (and DLR?) third rail, etc. 3000 volts (double again) is in extensive use in central and eastern Europe.