When running on the overhead wire, the trams used an earth return via the wheels and rails. (in fact in event of a derailment where the trolley pole still beon the wire, the crew were instructed to jump from car to the ground rather than create an earth when dismounting!)
When running on the conduit the system supplied both positive
and negative. Two subsurface 'T rails' were contacted by the plough which took the current via the busbars to the tram's circuits.
As previously mentioned the system arose to avoid the unsightly overhead wires, but it was expensive to build and maintain and outer sections were wired instead.
A sectional diagram of the plough and T rail arrangement can be found at
http://londonmodeltramways.webs.com/ a website that describes my two 1/76 scale model tram layouts based on London's Kingsway Subway and Dog Kennel Hill.