Resources on old DC traction systems

Old DC traction systems have come up in a couple of conversations recently, both overhead wire and third rail, in USA, UK and elsewhere.

Here are a couple of old text books on the subject, now available free online:

Electric Traction by A T Dover, first published in 1917 but this is the 1954 edition.

Principles of Direct Current Electric Traction by Douglas W Hinde and Herbert E Ingham, published 1946.

I think only the third rail systems in Britain are DC. These are mostly 750v DC. There were some overhead DC systems but I am sure they have all been discontinued - converted to AC - except for some tram systems.

As well as the Southern DC network, which stretches from Weymouth to London and almost all the lines to the southeast, there is the third rail network in the Liverpool area. As they have some former southern vehicles there I assume they have the same voltage. There were a few other third rail systems: in Manchester; and in Newcastle but these have been discontinued. They were examples of the 1930s experimental try-outs.

The Newcastle system was replaced by the quasi Underground system, also using a DC system, but with overhead. The Manchester line was incorporated into the tram system, also with overhead DC.