"Raining heavily" would not be such an issue. The worst is when it just starts to rain (and has not rained for some time). In this situation, the tracks get extremely slippery, and an experienced driver would use the magnetic rail brakes to stop at the right point (just relase at the correct moment…).

However, I don't know whether those vehicles are equipped with magnetic rail brakes (should be because of the steep grade, but one never knows).

Especially if street (or kind of street) running is involved, operation will be "on sight", and not signal-controlled.

I guess there will be reports after the driver has given his statements to the authorities.