Railroad tracks are incredibly strong. They are composed of many tons of ballast (usually crushed stone) supporting heavy crossties (cement, steel or wooden timbers), massive steel rails (132 pounds per foot) on top of heavy steel plates with spikes that weigh one pound each. The structure is designed to last for years with very little maintenance but they do not last forever. Ballast that is saturated with water becomes unstable and does not offer proper support, water contributes to wooden tie decay, lack of lubrication leads to excessive wear of the moving parts of switches (and contributes to injuries), and the interaction of wheels to rail over time stresses the entire structure. Repeat: TRACKS DO NOT LAST FOREVER.