While I like the idea, I’d imagine it would be a bit of an overhead. Not only would each segment have to remember how long it had been since a train last rode over it, but every so often, the game would have to check each segment (which would add more overhead) and then change the track texture (or model even) with something more grassy. Segments with more than two exits (switches, crossovers, etc) would either have to be treated as a single segment (the rust/grass would only start at the next segment), or implement segments that can handle multiple levels of disuse depending on a particular track within the segment. Currently, a segment is either rendered as entirely low-speed or entirely high-speed even if the tracks are a mixture of low-speed and high-speed, so this would imply that the segments won’t be able to handle multiple levels of disuse unless the code is rewritten (this would also enable low-speed and high-speed sections to be properly shown).
As for rust being shown, as well as changing the texture or track-colour, you can also make it less shiny just by decreasing the specular component of the rails on that particular segment. While on the subject of specular lighting, when vehicles get older, their specular component decreases – that is, they become less shiny. If you replace an old vehicle with one of the same type, it would be nice to see it look shinier. That way you can get a feel of their age by just seeing how shiny they are.