Three-rail trams.

[G-CPTN]

The video:- http://www.youtube.com/watch?v=ui63-Zjbcvo
shows (IMO) London trams operating without overhead wires with a third rail visible between the tracks.
I'd never heard of this system before (though I am not a tram enthusiast).
A search on Googoo revealed a new third-rail system being installed in Boulogne in 2004 (though there is a sophisticated modern-technology safety system):-

A distinguishing feature of the Bordeaux network is the absence of overhead wires in the central area. When the original tram network was built, the city authorities adopted a conduit power supply for the tram network, to protect the visual image of the city centre as laid out by Baron Haussmann at the end of the 19th century, and this lasted until closure in the 1950s. When CUB decided in 1996 to reintroduce light rail, bidders were again invited to put forward proposals for propulsion systems which would avoid the need for wires.

Options considered included battery, flywheel or diesel powerpacks, but all of these would have posed operating restrictions due to the limited power availability. Eventually the choice fell on the Innorail APS (Alimentation par Sol) ground-level power supply system. Innorail was originally formed as a wholly-owned subsidiary of electrification specialist Spie Enertrans, but when that firm was acquired by Amec it sold its shares in Innorail to Alstom.

Getting approval of the APS equipment under new French safety-case regulations proved one of the time-critical factors in commissioning the network, according to Alstom Project Director Hubert Peugeot and Innorail Managing Director Antoine Picard. Test running was undertaken on an isolated 500m section in Lormont, which could be physically sealed off from other road users. After formal tests and an independent audit, the safety case was finally signed off on October 21, allowing commissioning trials on the remainder of the network to start barely two months before 'T-Day'.

Even before APS received its safety approval, it had already become clear that all three routes would not be ready in time. The CUB tramway project team decided to concentrate all efforts on completing Line A, and postponed the opening of Lines B and C to February. However, in early October one car was towed through the uncompleted junctions, across the city centre and out to Pessac. This enabled the start of gauging runs and commissioning on the overhead-equipped section of Line B from the outer terminus inwards.
Digital radio switching

APS relies on digital radio signals to energise short sections of a central third rail under the vehicles, providing a full 750V DC supply to each car as it passes. The first phase of the Bordeaux network has been fitted with around 10·5 km of APS, split into five sections over all three routes. As well as the city centre tracks, short lengths of APS have been installed on both eastern branches of Line A at the wish of the local communities. It is also being used on part of Line B, where the fire brigade wanted unfettered access to buildings along a narrow street.

The centre rail has two raised contact strips on the head, about 20mm wide, separated by a 10mm recess. The rail is laid slightly higher than the running rails, so that the pick-up shoes pass above the running rails at pointwork. The contact rail is split into 8m sections, separated by 3m glass fibre inserts to form an isolation gap. The power is fed via 980 switching boxes set into the track, each of which powers two adjacent 8m lengths.

The two sets of pick-up shoes on each car are carried at opposite ends of the single unpowered bogie, immediately beneath the pantograph. The shoes are spaced 3·2m apart to bridge the insulated gaps and ensure continuity of supply. The cars were initially fitted with cast iron pick-up shoes to help polish the contact surface of the newly-laid rails. After a few months the shoes will be replaced by softer carbon pick-ups to avoid excessive wear.

Changeover between overhead catenary and APS modes takes place while the vehicle is stationary at a stop. The driver presses a button to initiate an automated sequence which lowers the pantograph and extends the parallelogram-mounted shoes to engage the contact rail. Each car is also fitted with emergency batteries which will allow it to run for up to 1000m should it become isolated from the third rail or overhead supply.

To energise the ground-level supply, each LRV operating in APS mode repeatedly broadcasts an 8-bit digital message to an antenna under the contact rail. The radio message is broadcast as a conical signal no more than 100mm in diameter at ground level, to avoid any risk of cross-energising the adjacent track. Only when a switching box has received and identified the 8-bit message four times in quick succession will the relays energise the appropriate section of rail.

The positioning of the shoes and radio antennae is designed to ensure that the rail is only live underneath a vehicle. Changeover contacts on the track-mounted switchgear earth the centre rail to the running rails at all other times, to avoid any risk of a residual potential.

In the event of any fault, each switchbox is designed to fail safe and isolate both rail sections. An alarm will be triggered at the control centre, but the service should be able to continue with the cars coasting across the gap. The faulty switchbox will be replaced during the night, with quick-release connections for a rapid change-over. Defective boxes will be returned to the supplier's workshop for servicing.

APS will be used on 3·5 km of the Phase 2 extensions, including the bulk of the Mériadeck - CHR section where the fire brigade have again requested clear access to the hospital buildings.

The contract price for the APS equipment on Phase 1 was €1·5m per track-km, which is around three times the price of the conventional overhead sections. For the CHR extension the price has increased to €2m, although some observers suggest that the true cost may be nearer €3m. Picard says the price is likely to come down in the longer term as the technology is used more widely, and notes that Nice and Paris are among several cities that have already expressed interest in using a third-rail supply.

(from:- http://www.railwaygazette.com/news_v...e_garonne.html )

Another overhead-wire-free system is described here:-
http://www.railwaygazette.com/news_v...tion_tram.html

Sections of the Braunschweig system are also three-rail:-
http://www.railway-technology.com/pr.../braunschweig/
but these are all modern schemes.

Did London really have three-rail trams in the 1930s?

It seems so:-

The Glamorous Electric Trams
This heralded a major expansion into electric tramways. The London County Council (LCC) began taking control of the horse tramways in London in 1896, and by 1899 had taken over the principal lines in south London. They now began an extensive programme of electrifying the old routes. For this they adopted the 'conduit' system of supplying power by means of a live rail buried beneath the road surface, rather than the cheaper, but unsightly, overhead lines. The first of these new electric-powered conduit lines opened on 15 May 1903. It ran from Blackfriars and Westminster to one of the LCC's new cottage estates in Tooting, south London.

(from:- http://www.storyoflondon.com/modules...rder=0&thold=0 )

More on ground-level supply:-
http://en.wikipedia.org/wiki/Ground-level_power_supply

and conduit-collection:-
http://en.wikipedia.org/wiki/Conduit_current_collection

The last (London) tram was withdrawn in June 1952.
The last conduit line in Paris closed in 1936, while the last Bordeaux conduit car ran in 1953.
The conduit systems in Berlin, Vienna and Budapest were very short-lived. All three were replaced by overhead working before World War I.

[wilfo098]

You may have read or seen this elswhere but the lodon authorities at the time did not allow overhead wires inside a particular area . The London trams had a thing called a (like a knife blade ) plough which collected the current and returned it through the rails( Earth) . Frinstance if a tram arrived from the east end of London at say somewhere like Walthamstow a changer man would attach the plough under the tram and pull down the trolley pole for the journey on into the centre. The reverse would apply on the return journey........... Regards Alan

[G-CPTN]

Thanks. Until I saw the video - and did my research - I'd never heard about such a system.

I'd like to see drawings of the system (and see how they maintained contact with both 'rails' - or was it 'earth return' though the rails?).

Are (were) overhead wire systems 'earth return'?

[wilfo098]

Have a look here I have drawings some where and books the actual part that went into the ground looked like an upturned horseshoe with the current collector at the bottom .It was dc current and failsafe all the metal was at zero potential, live at the single pole or plough..Did you know that most tramways found it difficult to compete with other transport forms as most tram companies had to pay for the upkeep of the roadway for about 1yd(mtr) each side of the trackway..then there was lack of investment ie maximise profit,...fiddle with this link as found a couple of dead Links Regards Alan

http://dewi.ca/trains/conduit/ploughs.html

[Trapper John]

I had heard of the "3-Rail Tram System", which I knew as the "Trough and Plough", but I was always under the impression that the trough had a moving cable in it! As an engineer, I find it incredible that live electricity would be in a trough! It would be easy for the trough to fill up with everyday rubbish, but the biggest bugbear must have been water. If it rained, or there was a nearby mains leak, the trough would have filled with water. Surely any pedestrian in close proximity to the trough would have been electrocuted, or am I missing something?

[G-CPTN]

And don't even contemplate ice . . .

[wilfo098]

I may have got it exactly right for I now notice the 2 cables emitting from the top of the plough and contacts each side of the base. However most must have used earth rail return as they only had one roof pole(pickup) in London that is there are photos of the system under the road ao another page of the afore mentioned wed site......Alan

[kingsway john]

When running on the overhead wire, the trams used an earth return via the wheels and rails. (in fact in event of a derailment where the trolley pole still beon the wire, the crew were instructed to jump from car to the ground rather than create an earth when dismounting!)
When running on the conduit the system supplied both positive and negative. Two subsurface 'T rails' were contacted by the plough which took the current via the busbars to the tram's circuits.
As previously mentioned the system arose to avoid the unsightly overhead wires, but it was expensive to build and maintain and outer sections were wired instead.
A sectional diagram of the plough and T rail arrangement can be found at http://londonmodeltramways.webs.com/ a website that describes my two 1/76 scale model tram layouts based on London's Kingsway Subway and Dog Kennel Hill.

[Bobjork]

Originally Posted by kingsway john

When running on the overhead wire, the trams used an earth return via the wheels and rails. (in fact in event of a derailment where the trolley pole still beon the wire, the crew were instructed to jump from car to the ground rather than create an earth when dismounting!).

Ok, I jump in here to answer a few days after the last post

But as you say the return is through wheels to the rail.
When I was active on the museum/vintage/commercial tram traffic with old trams we learned exactly what you said. Jump of without touching the tram and then pull down the pantograph (or pull it down from inside the tram if possible) with the wire. Then mount the "växelspett" (metal bar with narrow end and a handle used for turning the point to change track) to the coupling and to the rail. Then pull up the panthograph again and drive off from what isolates the tram.
Of course, if you have derailed you don't need to drive away from there. Let the recovery crew take care of that.
But two axle trams (not bogies) can be isolated if there is a lot of sand or other isolating material on the rails.