We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember (Andrew
Gabriel) saying something like:

I'm not thinking of a trip of the breaker due to overload but due to earth
leakage.

The circuit is earth return (track), so you can't measure earth leakage,
as it's not insulated from earth.

I used to think so, but a flashover from rail to actual earth via a
branch I threw onto a rail convinced me otherwise. This was about 45
years ago on an 25kV OHL urban system.
Fwir, it was a hot summer day and the weather had been like that for a
week at least, so I assume the normal earthy arrangements were
defective.
Makes me glad I didn't hold on to the branch...