I thought that was the case C-H but I couldn't chase the info down. DJK makes a good point though - where do they get their power.
I think in the UK (as common elsewhere in the rail industry) the traction current and signal systems (where both are ac) are on different frequencies to prevent interference. I thought I read somewhere that the UK uses 83.333' Hz(?) in these cases, presumably solid-state derived.
Interesting that 16.666' is 1/3 of 50Hz - no interference protection there from harmonics.
