L Class Tram Works Car
Here's a file for a London Transport Tramways L Class Works Car. It is drawn full size so scale to suit your model.
Two of these vehicles were ordered in 1909 by LCC. they lasted until the end of London Trams in 1952. They were used to transport wheelsets and heavy items between tram depots.
The originals had a trolley pole for overhead current collection but latterly only used the conduit system between the rails.
You'll have to fabricate a suitable structure to support whatever wheels you decide to use or some sort of motorised bogie if it is to be self powered.
Enjoy
Jonny