Because everything has a life span, fatiuge life.
Last night at work (RailRoad Yard) the crew was moving cars in and out of tracks to take out dead cars/ damaged cars for repair. They moved 3 pair (6 cars) pasted the same manual switch and all was fine.
When the next train that came into the yard had to pass "That switch" again, it would not throw properly and left a gap about 3 inchs wide, wide enuff to allow the train to fall onto the ground.
Lucky the conductor seen the issue before it became a major operation.
The worm gear, very much like the steering box of a car, just simply and finally gave out.
