With good ties, good ballast support, good aline, level, and cross level, you can go just about as fast on 90 lb rail as you can on 136 lb rail. The big difference is that it will take a lot more work, and I do mean A LOT MORE WORK, to keep the track with 90 lb rail in good condition because the rail does not spread the load over as long a distance. Also, the lighter rail will be much less tolerant of poor tie condition and variances in support condition, same basic reason, the light rail does not spread the load as well as heavier rail. Stresses in the rail will be higher, so the fatigue life will be much shorter than for heavier rail.

A number of year ago a company, which shall remain nameless to protect the guilty, started running a 3 or 4 times a week unit coal train (in 263,000 lb limit days) down a branch that was in 80 lb rail, and had a 35 mph speed limt. They reliad the line with 90 lb rail and replaced about 2/3 of the bad ties, and gave it a 3 inch lift of new blast furnace slag ballast on top of the old cinder ballast. The management could not understand why the train still derailed about one trip out of four.

It was not just the rail, it was the whole picture that was wrong. First, the 90 lb rail was nearly 60 years old air cooled rail with a fair amount of wear, and lots of small internal defects. It was laid jointed, not welded up. It should have been cropped and welded. Second, they should have replaced ALL the bad ties. Thirdly, at teh very least the shoulders in the cinder ballast should have been plowed out or better, the whole track sledded such that there would be good draining shoulders at no less than 6 inches of fresh ballast on top of the cinders and under the ties. Drainage work was also needed in numerous locations. You got to keep your subgrade from developing water pockets. With all this, done, the performance would have been much better. Of course, they could have laid new 132 lb rail with the 2/3 ties low lift fixes and probably also kept the train on the track much better, but they would have severely shortened the life of the 132 lb steel because of the poor support.

To return to the original question: FRA track standards are completely silent on WEIGHT of rail. They deal with geometry, tie condition, and such things as rail end mismatch, but in no way relate allowable speed to rail weight.

That having been said, I'd like to add that any rail less than 90 lbs. per yard should be replaced (or the line abandoned). Reason: 286,000 lb. cars (36 ton axle loads) stress rail of 85 lbs. or less beyond its yield strength, so every car of that weight that operates will deform the rail. It's only a matter of time before stress fractures start to occur.

Complete agreement with you, except, if we are talking about trains instead of the random car, if you are buying new material, I would say buy 136RE at least. If used, go for 132 lb/yd or better. If you have 85 lb or less, if you can not get something at least 100 lb/yd to put in, it is probably not worth the effort to simply stick in 90 lb or another light weight rail. If you budget is minimal, you are better off spending it on ties and drainage work.