I had always been under the impression that return rails for electrified rail systems were safe to touch, since they are at ground. Certainly this has to be true for systems that use the running track as the return as these are crossed all the time at level crossings. But I'm confused about how this works on the London Underground. I understand that that system uses a fourth rail for the return (for various reasons, primarily so that the return can be properly insulated from the old cast iron tunnels) and had always assumed that these rails were grounded (at their "ends") like running rail returns and thus safe to touch; but I read recently that the return rail is at "210 volts below earth". Doesn't this mean that that rail dangerous to touch?
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 "return rail" 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.
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)
On the London Underground, the fourth rail is certainly not just a "return current" rail, and is nowhere connected to ground.
It is maintained at -210V, which combined with the +420V on the 3rd rail, gives the required 630V.
See the following section in Wikipedia:
On the London Underground, a top-contact third rail is beside the track, energized at +420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC.