Another example: most cars have cast-iron brake discs. The pad friction (pads usually ride lightly on the discs all the time so when you hit the brakes there's immediate action) keeps things clean. Wash the car and spray down the wheels, then (if you can see the brakes), watch them rust before your eyes. 1/2 hour is enough to develop a very noticeable rust layer, both by sight and (when you first hit the brakes lightly) by sound.

Now think of rail coming out of the rolling mill hot and, from rain in storage or tranport, or from cooling sprays during milling, damp. Think instant rust. Of course, on something as relatively massive as rail the rust isn't a structural issue; in fact it might be protective, as in "Cor-Ten" structural/decorative steel.