Electrically-powered vehicles need some sort of power source. Subways often use a third rail (a conductor rail) and most other rail systems use overhead wires. Where it's impossible to have wires overhead for technical or legal reasons, a few rail systems used
the conduit: London and Washington D.C. for example: see London Trams: Construction of a Conduit Tramway. I had thought
that the line described there would be the last conduit system ever, but the conduit principle is very much alive at the
Panama Canal. The New York Times of December 29, 1912 described the power conductors as being in an "open conduit"
but I suspect they meant the same system as is used now: a covered trough that has an open slot in its lid.

	A top view of the towing track: the dock wall (the water) is at the bottom of the picture: above it are the two running rails with the rack rail lin between, with the conduit slot above it.
	A close-up view of the conduit. The street tracks in London and Washington were buried in the paving stone except for access hatches. The Panama Canal conduit has a continuous cover which can be unscrewed and lifted at any point. Of course, the Canal has the advantage that no other traffic has to ride over the conduit cover.
	The plough or plow which extends down into the conduit, and which has current collector shoes which press against electrical power rails. Each wheel bearing on the side of the mule away from the water has a plough/plow.
	Another plough/plow. There are two wires, which suggests a DC system or a single-phase AC system.
	Two years before the above photos were taken, many of the mule tracks were relaid. Jamie Guest took photos of the relaying and has kindly allowed me to post a photo here. Jamie runs a website http://leedshorsecar107.squarespace.com about the restoration of a Leeds horse-car, one of the earliest trams to run in that city.
	This is a close-up of part of Jamie Guest's photo. On the left are the running rails with their cross-ties. On the right, the green bars appear to be temporary supports to hold the metal edge-rails of the conduit while they are installed. In the conduit itself, there are yokes similar to the ones used in London