On a railroad I worked on, the line ran along a river for most of its length, and it was under and near trees. Especially on cool fall mornings, the combination of condensation and wet leaves on the rail set up any unsuspecting engineer for trouble. You'd be rolling along in the third or fourth notch and suddenly notice the speedometer was showing 35 mph and you were slowing down. All eight wheels of the engine would be slipping on the leaves without tripping the wheel slip relay, and you often had to come to a complete stop before you could gain control of the spinning. Sometime, even with sand, it was very hard to get moving again. on more than one occasion, I had to have the conductor stand on the front step and hold a switch broom on the rail to sweep the leaves away and give us half a chance of making forward progress!