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In depth idiot
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From appearance I think this must have started life as a three rail mechanism, (the plate from which the valve gear is hung bridges the driving wheels) which suggests to me that the wires are 'flying leads' to run the motor with no power collection from the rails. 'Quick and dirty' test, try a PP9 battery on the paired green and brown wires to see if that makes the motor run: green to one battery terminal, brown to the other.

Not being able to see what connections there are to the pick up skid (if present) or motor brushes, it is difficult to suggest anything further.
 

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In depth idiot
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Side question out of curiousity. I only have an 8F 3 rail chassis to look at, (no motor or wiring) and similar to the Duchess, the steel plate at the front end which supports the all metal valve gear means there is a conduction path from coupled wheelface to wheelface - which is fine for 3 rail of course.

So to the question, for 2 rail where did H-D have insulation to prevent the side rods shorting out the rails on outside valve gear models? As far as I can see, on the wiper pick up side it would have to be every crankpin insulated from the wheel.

... I can't go and take a body off one to check how the thing is wired up...
Same here, but that pair of green and brown wires loosely twisted are not original H-D construction in my opinion, thus my guess that the chassis is modified to allow operation on unpowered track, supplying power via that pair of wires. And if it is modified, then it's anyone's guess what has been done. We won't know more until Alastair posts.
 

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In depth idiot
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Below is a photograph of my City of London which is, wiring aside, identical to the one in the photograph in first post in this thread...
That's helpfully clear. (For Alastair, the blue wire is the insulated feed from 3rd rail skid or 2 rail pick up wipers, yellow insulated component in line to the insulated brush is a choke, and the small grey capacitor bridges the insulated supply and the live to rail chassis block via the terminal just forward.)
... Your thought of the difference maybe having something to do with the coupling rod pins may well be right, as the coupling rod pins on the insulated wheels had a different part number (20689) from the coupling rod pins on the uninsulated side, which had the same part number as all of the coupling rod pins on the three rail version (7741)...
Thanks for confirming that. Couldn't see any more economical way of achieving this given the constructional plan.
 
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