Re the big Building in Waldram Place, now a MOT testing station.

Yes I think you are probably right saying that it was for the electricity supply for the trams. Not a power station generating electricity from coal etc, but I can remember the sound (and smell) of electrical machinery from there so it was probably converting Alternating Current from the national grid into Direct Current for the tram motors.
Any tram experts help us on this?

So are you going to keep us guessing about the location of this mysterious tramway that will be the subject of your book?