I want to know the answer to the following question:
What is the voltage of the return rail?
All the background I'm provided with is that "On the London Underground, a top-contact third rail is beside the track, energized at +420v DC, and a top-contact fourth rail is located centrally between the running rails at −210v DC, which combine to provide a traction voltage of 630v DC.". Is that enough to answer the question?
Answer: yes