Working on my local heritage railway, it's a well accepted fact that locomotives will slip more after rain, when the rails are still damp, than during even the most torrential of downpours - trains can tackle a steep hill in the rain, but have trouble getting moving whilst on the level later on.

However, I can't for the life of me see why this would be the case - on the road, its usually riskier when it is still raining and there is more water on the road surface. I would have expected a similar effect for trains, but the opposite seems to occur!

Does anyone have some thoughts as to why this happens?

Edit: Now you've all said it, I do remember having heard that partly-wet roads are worse than very wet ones, excluding the chance of hydroplaning.