Change third rail to bottom contact

[rebmcr]

Was a centre live rail ever used anywhere (like my old Hornby Dublo!)? I've often wondered why third rail electrification was always 'outside third', never 'centre third'. Of course I know about "Outside third, centre fourth" on parts of the Underground, and it crossed my mind that centre fourth seems to work OK there, so why not 'centre third' as the live rail?

Reduced chance of making the running rails live through pointwork?

 

[jopsuk]

I'd reckon pointwork for one, and train maintanence for another (choebeam on bogie is easy to access), are the main reasons.

(For truly esoteric electical systems of model rail systems, the LEGO 12V trains. 3rd and 4th rail both in the middle, on a single component)

 

[joeykins82]

There may have been a public safety consideration too; falling from the platform on to the tracks where the 3rd rail is in the middle of the tracks is probably going to result in electrocution and death, whereas having the live rail by the side of the tracks away from the platform reduces the likelihood of that happening.

 

[Gareth]

Yes, the live rail can always be furthest from the platform. The pick up show would also have to be underneath the train which would make maintenance a bit of a bummer.

 

[edwin_m]

A centre rail looks logical - if nothing else it would put the live rail and shoegear underneath the train where it is less likely to be touched. One problem is that the centre rail would have to be interrupted on pointwork, whereas an outside rail can hop over to the other side except in very complex layouts.

The reason may be because the Underground got there first and put the positive rail on the outside (except on one line IIRC). For complicated reasons to do with corrosion it's better to have the running rails as negative so when the LSWR (?) decided they would have a nearly compatible system but with only three rails they were moreorless forced to use an outside rail.

 

[JGR]

It doesn't really matter which way round you run the current, the issue with LU is that the tunnels are cast iron (if I remember rightly), and the running rails are bonded to, or at least poorly insulated from the tunnel. You'd get galvanic corrosion and other unpleasant issues in the tunnel walls if the running rails were used for the return current.
It's not as much of an issue on the mainline as ballast doesn't have that problem.

I'm not entirely convinced by the track safety argument. If you fall onto an electrified railway all bets are off either way. Also, away from platforms, the conductor rail is usually on the outside edge on both sides (on double track), (for ease of maintenance access, presumably), making it the first rail a person (whether legally present or otherwise) is going to come across.

 

[edwin_m]

It doesn't really matter which way round you run the current, the issue with LU is that the tunnels are cast iron (if I remember rightly), and the running rails are bonded to, or at least poorly insulated from the tunnel. You'd get galvanic corrosion and other unpleasant issues in the tunnel walls if the running rails were used for the return current.
It's not as much of an issue on the mainline as ballast doesn't have that problem.

Running rails are nominally insulated to allow track circuits to operate, but still have quite a bit of current leakage either on Tube or ballasted track when wet or old. The third and fourth rails sit on pots so the insulation is much better. IIRC electrolytic corrosion happens at the positive pole hence why the live rail or wire is positive on DC systems.