Our Railroad operates on Track that is 70lb rolled in 1905 and we run 286,000lb cars on it.
It is tested each year and we find very few defects. With that said in 2005 the area with the 70lb rail was re-tied with alomost 100% new ties and then ballast was added and the track raised 4 inches and tamped. It is tamped as needed each year. We also run on 75lb rail and 90lb rail and have the most trouble with defects in the 90lb rail due to the way it was processed at the steel mill with more internal defects 1931 rail. these tracks have alos been tied and tamped and handle 286 cars. The main line is 110lb rail all the way up to 141lb rail rolled in 2004 and all of it has new ties and ballast and is testes twice a year. The 110lb rail is alos prone to internal defects.

The truth is with good ties, ballast and most improtant good drainage you can operate 286000lb cars and large six axle locomotives on small rail. You need to test rail at least once a year for internal defects. Rail or 90lb with good ties,ballast and drainage with good surface is good for speeds 40 MPH and it gets tested twice yearly. While we would all like to have 136 to 141 new welded rail and concrete ties in the shortline industry if you maintain your track to better than FRA minimums properly and on small rail keep speeds at 10 mph and test for defects and inspect all tracks weekly you can run bigeer cars. It the same with bridges, culverts and tunnels you must maintain them. some shortline have bridges that are not built to handle 286 cars but can still run the 263000lb cars.

Its all about having enough traffic to support maintaining the track, signals, bridges and pay the folks a fair wage and make a small profit at the end of the year.