One of the first major manufacturers to exploit this dual train facility in 00 scale was the aptly named Trix-Twin system. The main drawback to their system was that their own special track was required which had an extra centre insulated third rail which was not readily compatible with other manufacturers track. In fact Trix-Twin even exceeded their own expectations by becoming Trix-Triple, introducing their own catenary system thus allowing no less than three trains to be independently controlled on one section of track. On most real electric railways though, power is usually supplied by an outside third rail or overhead catenary and returned trough the running rails. It is the latter that Tri-ang Railways introduced in the 1960's.
Tri-ang overhead electric locomotives were equipped with fully functioning pantographs connected to a three position selector switch allowing you to collect power from the track, a neutral centre off or overhead catenary. Power was returned via the other common running rail as usual. The clever bit is how this facility was promoted and kept simple. Tri-ang used the term "Hi/Lo Dual Control" for this feature and stated that no complicated electrics or modified locomotives were required. Remember that in this era, toy or model trains were intended for a young clientele to play with and enjoy, not like the situation today where everything is obsessively scrutinised by self nominated mature gentlemen and the model railway hobby is taken far too seriously.
One line controller is connected to the track in the normal manner and the other is connected, one wire to the common return rail and the other to the power feed mast on the catenary system. There is no difficulty having two electric locomotives on the line as long as one is switched to the track and the other is switched to overhead. However, the one working on the overhead must be orientated the correct way round. There is a little "R" indicating return or common on the bogie frame to assist in identifying this.
If you study the associated Tri-ang Railways wiring instructions you will see just how simple the whole system is to set up and play with. Anyway, it's much easier than reading through this article again.