Third rail: Encyclopedia II - Third rail - Compromise systems

Third rail - Compromise systems

There are and have been several systems in which third rail has been used for part of the system, and overhead lines for the remainder. These exist sometimes because of the connection of separately-owned railways using the different systems, or because of local ordinances.

In New York City, electric trains that must use third rail leaving Grand Central Terminal on the former New York Central Railroad (now Metro-North Railroad) switch to or from overhead lines when they need to operate out onto the former New York, New Haven and Hartford Railroad (now Amtrak) line to Connecticut. The switch is made "on the fly" controlled from the engineer's position.

The Blue Line of Boston's MBTA uses third rail electrification from the start of the line downtown to Airport, where it switches to overhead catenary for the remainder of the line to Wonderland.

The older lines in the west of the Oslo T-bane system were built with overhead lines (some since converted to third rail) while the eastern lines were built with third rail. Trains operating on the older lines can operate both with third rail and overhead lines.

In Manhattan, New York City, and in Washington, D.C., local ordinances required electrified street railways to draw current from a buried third rail accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead. Some sections of the former London tram system also used the conduit current collection system, and here too there were some tramcars which could collect power from both overhead and under-road sources.

Several types of British Railway trains operate on both overhead and third rail systems, including the class 313, 319, 325 and 373 Eurostar trains.

The newly built Tram in the City of Bordeaux (France) uses a novel system with a third rail in the center of the Track. The third rail is separated into 8m (25ft) long conducting and 3m (10ft) long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the train (activated by a coded signal sent by the train) and switch it off before it becomes exposed again. This system (called "Alimentation par Sol" (APS), meaning "current supply via ground") is used in the historic center of Bordeaux: suburban line segments use a conventional overhead line.

Adapted from the Wikipedia article "Compromise systems", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki