Yesteryears Conduit Operation Unnecessarily Expensive?

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.

I share your wonder about London and Manhattan, but may have an explanation about Washington. An ungrounded system (double overhead wires) was required in the District of Columbia outside the City of Washington because of the Naval Observatory; maybe this also applied in the City, where conduit was required? When the observatory moved to Annapolis, the Washington, Baltimore & Annapolis had to use an ungrounded system in that place. The Annapolis Short Line, the other interurban there, may have been a steam road at that time.

Actually, Washington was inconsistent. Most lines that extended beyond the old city limits used a conventional single-wire overhead. This included the Wisconsin Ave. line, which came closest to the Naval Observatory. Double wire was used only on two lines on the east side of town -- the Anacostia line and the so-called Columbia line to Seat Pleasant and Kenilworth (which the WB&A used as its DC entry.) The part of the Columbia line that used overhead was almost entirely on p.r.w., by the way. Offhand I don't remember when the double wire was given up, but it was probably in the early 1920s.

JimBoylan wrote:. . . . . the Washington, Baltimore & Annapolis had to use an ungrounded system in that place. The Annapolis Short Line, the other interurban there, may have been a steam road at that time.

Actually, the WB&A and the Annapolis Short Line are the same property. There were two underlying steam railroads which were WB&A predecessors--- The Annapolis & Elridge, which later became the WB&A's South Shore route into Annapolis and the Annapolis & Baltimore Short Line, which became the WB&A's North Shore Route. The South Shore route was abandoned, along with the Baltimore- Washington Route in 1936, when the bankrupt WB&A was sold at auction in Annapolis. The North Shore Route continued operation, as the Baltimore & Annapolis Railroad until 1951. Part of THAT route is now the southern portion of Baltimore's Central Light Rail Line.

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.

Here's a link to a photo of a plow used on the DC system, on display at the National Capital Trolley Museum:

http://dewi.ca/trains/2004/NCTM/pict0012.jpg

100 Years of Capital Traction by LeRoy O. King Jr. states that the two rail conduit system was used to prevent electrolyses damage to nearby buried iron pipes from stray current.

A neat little feature to the two rail system was that the hot and return currents could be swapped between rail when stray current were detected.



(Left) DC Transit girder rail salvaged from U Street NW near 8th Street NW during WMATA construction.

(Right) DC Transit contact rail salvaged from 14 Street NW near Irving Street NW during WMATA construction.

When I grabbed the section of contact rail there was an intact yoke in the rubble. I considered saving it, but it was incrusted with so much concrete that was to heavy to lift into my vehicle.