Its a system for powering trolley cars. Since the power demand of traction cars is low compared to locomotives pulling trains it is possible to run traction cars on half-wave control.
If there are three conductors, the two running rails plus a common overhead, two independent two-controller half-wave systems can be installed giving independent control over four cars. That is what is meant by the "Detroit System."
Reference: Model Traction Handbook by Paul Mallery & Steven Mallery (1974).
Has this been done with 3 rail instead of overhead?