Yoou are quite correct in concluding that the "downstream" position of the trolley frog acted as a self-orienting point for the trolley wheel. Note that this worked for wheel pole systems; if you used a "slider" then I think it needed a positive switch. Incidentally European practice is to have a radio-controlled transmitter in the trolley motorman station; the motorman simply pushes a button to activate the switch points and the trolley frog. Some trolley and bus routes there are on reserved right-of-way; the driver just activates the radio signal system to lower a sturdy steel pillar fitted to a hydraulic piston to clear the roadbed; the big "pin" seriously deters motorists tempted by the empty lane!