Disney Guy wrote:Perhaps some transit historians might want to comment:
Why, when underground conduit power for streetcars was used (New York City, Washington DC, and many other cities), did they not have just one rail underground for the positive or hot side and the track used for the ground return?
Having both positive and negative rails underground seems like unnecessary expense, both for construction and for maintenance.
To actually answer this question, in an underground conduit system, at least in the DC system, the slot rail and the two running rails were attached to 350 lb yokes set in concrete at five feet intervals in the street. A conduit ran beneath the slot rail through which ran two "T" shaped conductor rails which were supported by insulators hung from the bottom flange of the slot rail. A plow which was suspended from the truck of the car had two sliding shoes which picked up positive and negative current from the two rails. I suspect that was the need to use these insulators which precluded running the return current through one of the running rails, since both running rails and the slot rails were part of a single assembly. Without this insulation, there would be a signinficant leakage of current into the street. Though I don't really know why, apparently this was not as big a problem for running the return current through one of the running rails in an overhead system.
One interesting facet of the DC operation was the fact that throughout the streetcar era, the plows were manually installed and removed from the cars at the locations ( known as "plow pits") where the cars went from the conduit system to the overhead wire system. There was a chamber in the ground at these locations ( the plow pit itself) in which was stationed an employee who installed the plows on in bound cars, and removed them from outbound cars. There was a break in the current flow at the plow pits to permit safe installation and removal of the plows, which may be another reason why return current was not run through the running rails.