The picture accompanying the Shore Line Interurban Historical Society, April calendar page features a ten car South Shore train somewhere in Indiana. Looking at that picture got me to thinking about how the overhead direct current electrons flow out of each car.
Now I think we can all agree that all of the steel wheels attached to each truck pass electric current to the rails, but my specific question is this: how does the current actually pass from the carbodies and traction motors to the wheels?
Electric current passing from the body bolster to the truck bolster, and through the spring group to the truck sides, and from there to the journal box would encounter a significant obstacle within each journal box: that thin film of journal oil that lies between the wheel brass and the axle. In fact, if 600-volt, high-amperage electric current were to try to breach that thin film of oil, it would seem to me that there would be sparks flying from journal boxes and fires would ignite from within many of them.
With roller bearings the situation wouldn’t be any better. They contain a petroleum based lubricant and likely would be subjected to tremendous internal pitting.
So how exactly did the builders like General Electric, Westinghouse, Brill, Jewett, Cincinnati, Saint Louis Car, the boys in the electrical shop at PRR/Altoona, and others get around this problem?
They use a carbody wire to a carbon brush either on the axle end or as a ring assembly on the inner axle, as can be seen in this shot of a P32acdm. the ground brush is on the left axle bearing.
There better NOT be 600 volts on the carbody! (At least, not normally.)
The new dual-mode deisels may have brushes and ground wires, but as far as I know and have seen, under streetcars and older interurbans, the electric current simply flows down the frame, through the bolser, and from the wheels back to the track, and into the ground. Oil is conductive - transformers sit in a bath of oil to prevent sparks - to get a spark, you need a gap of air that the electricity has to jump through. The friction bearings aren’t much of a problem at all.
There shouldn’t be a lot of current in the carbody. The only electricity up there should be contained in control wires, etc. There might be a small amount of grounded current for the lights, possibly the heaters, etc that was simply grounded to something metal inside the ceiling. Someimes current can flow through the carbody - which is why on rusty rail, or during heavy pulling/towing moves, when the ground through the r
