3rd rail vs 4th rail current systems

[Deleted]

I'm curious...is there any advantage of getting rid of the 4th rail in the tube and run it on a more "conventional" 3rd rail system?

I would think with less metal and parts, it would save on maintenance in the lond run. My very limited impression is, running new stock on 3rd rail alongside older 4th-rail trains being replaced isn't too hard because of what I read about the old Bakerloo Line:

When tube trains use this stretch of track, the central rail is at a potential of 0 volts, and the outer rail at 650 volts so that Underground trains can use the same traction system as the mainline trains. London Underground trains normally run with the outer tail having a nominal voltage of +420 Volts and the inner rail running at -210 Volts. The potential difference of 600-650 volts remains the same though so the trains operate normally.
underground-history.co.uk/croxley.php

So is there any special reason why LU's rather unusual configuration of quad-rails is used?

[Oracle]

I query whether the conversion from 4th rail on the DC lines, including the Croxley branch and the two DC lines into Euston, plus of course into Broad Street, was a backward step as regards traction supply? However it clearly meant savings! I always thought, and I am sure that this is wrong, that the 4th rail system was better-insulated as regards earth returns and that the system is required on the LU because of potential electrical leakage in the tunnels.

I cannot recall if 4th rail was used anywhere else in Great Britain. Outside the former Southern network, 3rd rail was used on Tyneside, plus the Liverpool system (and Liverpool Overhead Railway?), and Manchester-Bury used a side-contact 3rd rail.

[mrfs42]

Partly historical - there was a legal requirement to have less than a 7 volt voltage drop along the line to prevent electrolytic corrosion of underground services in the surrounding area (pipes, gasmains et al); the fourth rail also allows services to continue if one side of the supply develops a fault to earth (this would just blow the breakers on 3rd rail) and also the traction current can be completely divorced from signalling current with 4 rails - removing the 4th rail would require an AWFUL lot of suppressed relays putting in for the signalling, probably at net higher cost than at present for maintaining the 4th rail.

No doubt there will be other more knowledgeable bods along in a bit to give you a bit more of an explanation.

[Deleted]

just a note i dunno why its been like this but when the ELL upgrade took place in the 90's it was wired for 3rd rail only in the substation aswell as 4th but other then that i dont know anythink about the traction supply

[Oracle]

I have been prompted to remember where LU 4th rail was once installed and is no longer extant.

My immediate recollections are:

1. Croxley branch as far as west of Crixley Depot then four or so sidings in the depot
2. Croxley Tip siding
3. From Tubeprune the Up Widened Lines as far as Moorgate...was there 4th rail on the connection between the CWL and the Inner Rail at Faringdon? When last used it was unelectrified.
4. Acton branch
5. Hounslow West old station
6. Hounslow Town branch
7. Shoreditch
8. Epping-Ongar
9. Northern City Moorgate-Finsbury Park
10. Harrow & Wealdstone to Watford Junction

Off the Southern network when was the last electrification with 3rd rail? Moorgate branch?

[cetacean]

oracle said:

Off the Southern network when was the last electrification with 3rd rail? Moorgate branch?

NLL to North Woolwich in the early 1980s?

[Deleted]

oracle said:

I cannot recall if 4th rail was used anywhere else in Great Britain.

I also can't recall it being used elsewhere in the UK, although possibly some UK theme park monorails and airport AGTs may use +ve and -ve current rails.

Outside of the UK, it is used on a few other metros, but not in the same configuration as London. Most Bombardier ART systems have the 3rd and 4th rail close positioned close together. The Paris style pneu lines have +ve and -ve rails - this system is also used in Montreal, Mexico City, and Santiago. Quite a few automated guided transit systems, and monorails also have +ve and -ve current rails.

[metman]

In addition: Parts of the Alexandra Palace branch also had conductor rails installed but never used, the Watford High Street to Croxley Green and Rickmansworth (Church Street) and the entire Broad Street to Richmond service used 4th rails until 1971 although not part of the LT network. It has been mentioned but the North London lines used class 501s with 4 rails until the 501s were converted to 3rd rail in 71.

What puzzles me is that in many locations the 4th rail exists but not on rail chairs and bonded to the running rails! Why is this-I've seen it on the Thameslink at Farringdon and Blackfriars and I think bits of the Northern City and North of Watford Jn?

[Oracle]

11. Edgware former (truncated) line from Mill Hill (The Hale), used as sidings

[Rich32]

metman said:

What puzzles me is that in many locations the 4th rail exists but not on rail chairs and bonded to the running rails! Why is this-I've seen it on the Thameslink at Farringdon and Blackfriars and I think bits of the Northern City and North of Watford Jn?

Certainly on the Northern City, this is for the current return to go back through the 4th rail to prevent the aforementioned potential electrical leakage and consequential tunnel corrosion.

[metman]

That figures! The Thameslink goes thru the Snow Hill tunnel. Thanks rich

[Deleted]

stephenk said:

The Paris style pneu lines have +ve and -ve rails.

Not so: the Paris Metro is three-rail. The live rail is outside the track, roughly level with the concrete guides for the horizontal pneu wheels. Beneath the train, as well as the concrete tracks for the pneu wheels, there are steel rails for safety. If a pneu tyre blows, a nromal steel wheel runs on the rail. These safety rails are used for current return, with shoes pressing down on them. Which is why they are clean and shiny.

The District (and perforce the Met) used 4-rail because (a) it was what the American financiers were used to, and (b) it eliminated problems of electrolysis caused by current leakage. For the tube lines, there was some concern about electrolysis of the tunnel linings, but more important was that it did not matter if the outside rail shorted to the tunnel lining. When that happened, the outside rail was at 0 volts and the centre rail at -600, and the trains kept running.

[tubeprune]

sydneynick said:

stephenk said:

The Paris style pneu lines have +ve and -ve rails.

Not so: the Paris Metro is three-rail. The live rail is outside the track, roughly level with the concrete guides for the horizontal pneu wheels. Beneath the train, as well as the concrete tracks for the pneu wheels, there are steel rails for safety. If a pneu tyre blows, a nromal steel wheel runs on the rail. These safety rails are used for current return, with shoes pressing down on them. Which is why they are clean and shiny.

The District (and perforce the Met) used 4-rail because (a) it was what the American financiers were used to, .....

Er no. They used the 3-rail system in Chicago, New York and Boston. Most of the Underground's technology is based on what was done in those cities. Also, the C&SLR and Central London both had 3-rail. The Central London's lasted until 1939.

The most important reason for the 4-rail system was cost. The 7 volt limit on drop would have made it necessary to build more substations - very expensive. Together wiith the signalling, shorting and insulation issues, it was a no-brainer.

[Deleted]

sydneynick said:

stephenk said:

The Paris style pneu lines have +ve and -ve rails.

Not so: the Paris Metro is three-rail. The live rail is outside the track, roughly level with the concrete guides for the horizontal pneu wheels. Beneath the train, as well as the concrete tracks for the pneu wheels, there are steel rails for safety. If a pneu tyre blows, a nromal steel wheel runs on the rail. These safety rails are used for current return, with shoes pressing down on them. Which is why they are clean and shiny.

The side rail is +ve, the emergency running rail is -ve.
www.navily.net/composition_metro.php

[Deleted]

I also think 3rd rail system is safer if someone fell on to the track or for maintenance workers

[mrfs42]

(wandering slightly off-topic)

There was a short stretch of the Central Line with 5 (yes, FIVE) rails - I quote from the signalling notice from 1946 for the opening from Stratford to Draper's Field:

'The positive current rail is normally located on the left-hand side of the track in the direction of running. There are, however, a few instances in the tunnels between Stratford and Leyton where this rail is changed over for short distances to the opposite side of the track for structural reasons; also between Bethnal Green and Mile End stations for a length of 1,740 feet in the westbound tunnel and 1,800 feet in the eastbound tunnel, the positive current rail is installed on BOTH sides of the track. THe site is approximately 690 feet west of Mile End where the line is in the vincinity of Queen Mary's College.'

Why? I'm not altogether sure - presumably is was an effort to stop any current leakage affecting any experiments going on in the college.