Steel is a product of combined processes and has a lifespan. This can be determined, among other things, by composition, use and care. As an apprenticed welder, then Carman and now a trainman for the past nine years, I can assure you that old steel (in this case rail) will suffer the consequences of applied dynamics.

Three years ago, I was part of a crew that kicked a load of feed into a track that had old, light rail; probably 7540 something similar. The car it hit was bullheaded, and when it bounced off, it went on the ground. The joint it picked to destroy was work-hardened from the web up
and broke a piece about 14" long into six pieces.

I think the term is magnatite (SIC), as referencing to the crystalized grain pattern that occurs in work-hardened steel.

The ability to handle heavy loads under high strain/impact should be a hallmark quality of rail that we can bet our lives and litigation on.