Norfolk & Western Elkhorn Tunnel
Based on street railway practice it may well be a dynamo car.  They filled the same function as fixed facilities to step-down and convert high AC transmission voltages to the lower DC line voltages required
by the motive power. This was accomplished with a dynamo made up of an AC motor turning an armature spinning inside a set of coils (windings) to convert the field to DC.  As may lines changed over to AC motive power, which just required a voltage step-down through transformers, the name dynamo just stuck. If you look closely at the roof line of the car, on the right hand side, you can see three insulators.  They are for the drop from the AC transmission line.  Too bad we didn't have a picture of the other side.  It should have the hook ups for the trolley wire distribution cables.  The mobile units would be used when a fixed facility was off-line for repairs or maintenance.  In street railways they would be set up on seldom used lines, say out to the fair grounds or ball park.  There was no need to spend the money for a fixed dynamo house, when a mobile unit could fill in for the few weeks the line was used each year.
Caption data from J. D. Lloyd Back to N&W page
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