And where will the fourth rail pickup shoes be?
Usually under the front bogie.
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If you don't want to allow current leakage to the running rails when that bogie does not have a pickup shoe in contact with the 4th rail, you will need some sort of bus along the train connected to the fourth rail pickup shoes. (Doing so would largely undermine the entire purpose of the fourth rail in the first place.)[/QUOTE]
The return path to the centre (4th rail) pickup shoe is insulated from the vehicle chassis and wheels so if the shoe looses contact and there is no bus then the train looses power until the shoe regains contact. Exactly the same as for the 3rd rail shoe at pointwork or other gaps.
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As this bus carries the same current as the third rail pickup bus it must be of similar size.
Therefore the only way you can convert a train to fourth rail operation without requiring a net addition of bus lines to the train is to halve the current flowing through the traction package.[/QUOTE]
The current flowing in a circuit is the same at all points in the circuit. If a current of 1000A is required to drive a train from a 750V power source then 1000A flows from the 3rd rail shoe to the power equipment and 1000A flows back to the 3rd rail or the 4th rail. How the return current gets to the return path doesnt affect the current demand or the voltage required. The only difference between 3rd and 4th rail operation is the termination of the return cable on the shoe or the axle/wheel and the isolation of the return side of the current path from the vehicle chassis..
[/QUOTE]The only way to achieve this without reducing available power is to double the voltage between the 3rd rail and 4th rail compared to between the 3rd rail and running rail in a normal system.
This means +750V and -750V.[/QUOTE]
The class 503s ran for 18 years (from 1938 to 1956) changing from 3rd rail operation on the former Wirral Railway (electrified on the 3rd rail) and then during the station stop at Birkenhead Park switched to 4th rail operation so they could operate on the Mersey Railway with no loss of performance. Both railways operated at nominal 600V.
[/QUOTE]Converting a main line train to 4th rail is an engineering nightmare.[/QUOTE]
The Mersey Railway managed to convert their trains in 1938 to operate off both 3rd and 4th rail systems with no difficulty. Seems odd if it is/was an engineering nightmare.