Is it safe to touch the return rail on the London Underground?
Yes, if the return rail is 210 volts off earth, it is energized and dangerous to touch unless known to be de-energized under London Underground safety procedures.
It being a &quot;return rail&quot; in name does not overrule the fact that there's a potential difference of 210 volts from it to ground, which you will be subjected to if you step onto it while touching anything grounded. It only means that (conventional) current from the train's fourth rail shoes returns to the power substation through this rail.
A simplified schematic which ignores ancillary loads which may be referenced to running rail ground, is shown below.



simulate this circuit &ndash; Schematic created using CircuitLab
Of course, railways present other dangers unrelated to electricity, even if you're touching only running rails or ties. The running rails being at ground potential is not an endorsement to enter the tracks, except when certified (whether on a live mass transit system or at an operating rail museum as I have experience with)