From what I experience on the subway and tram on rainy days is that starting from a stop is also tricky, since steel wheels on steel tracks have not a lot of grip on rainy days, leaves make it worse, so the wheels spin in place and it feels like a slow, rocky start.
So I figure they also drive a little slower overall not miss the stop.
My city is pretty flat, so I’d guess that they don’t need all powered axles? In the subways it happens more frequently on the longer trains, that are full, so during peak hours.