Assuming that there isn’t a wire sticking into the water nearby, it should be at ground potential. Just stay away from the catenary wire or anything hooked to it, as that’s carrying (in Germany at least) nominally 15 kV.
While third-rail systems, like main lines in southern England, and most metro rapid transit networks, aren’t going to work under water, I’ve always been surprised at how few problems they have in normal rain conditions.
