Ever notice how waxy new-fallen leaves feel? Well, when they break off the tree and when they hit the ground, minute amounts of that wax are broken loose to cover every thing. Multiply that by millions of leaves and pretty soon you're talking about quite a waxy buildup.
Now trains obviously run with steel wheels on steel rails, which sounds like there wouldn't be a lot of friction to create traction. But with the high concentration of weight on a very small point of contact, it works. Older diesel locomotives have an adhesion ratio of about 25%, meaning that the wheels won't slip until the tractive force reaches about 25% of the weight on the wheels. Newer locomotives with more sophistocated anti-wheelslip systems can reach an adhesion ration of as much as 40%. But in the fall, add a little water to that waxy buildup on the rails, and adhesion can drop below 10%. It can get real fun trying to haul a freight train up a hill this time of year. I've had instances where just putting the locomotive in gear, without even opening the throttle, has caused runaway wheelslips. Look out the window and you're barely moving, but look at the speedometer and it says 40 or 50 or more. Kind of like trying to drive a car on glare ice.
Leaf wax plus a little rain or frost. I'm telling you, it's better than Teflon.
Now the funny part is that every year, this comes as a surprise to the railroad. Every year, the leaves fall. Every year, some time during that 2-3 week period when the leaves are falling, it's either gonna be mild and rainy or cold and frosty. And every year, for the first couple days like this, trains stall on the hills. Every year, the railroad reacts by adding more locomotives to the trains. But even so, the engineers have to be light on the throttle, as too much torque just causes more wheelslip. So every year, the dispatching office wants to know why we're losing time in the hilly sections. The only part about this that surprises me is that it surprises the railroad ... EVERY YEAR!
Wait 'til they see what falls out of the sky in December! And that will be just as much of a surprise!