Without starting a rail war I'd like to weigh in on my experience with rail sizes and uses.

I've worked with everything from 56 lb. to 141 lb. over the years and while no expert I think I have something to offer.

I've seen SD45's and unit grain trains on 75 lb. rail. It frequently didn't end well but it did work. The sidings were 56 lb. and these engines and cars traversed them and somehow managed to stay upright at least some of the time.

I think 90 lb. rail works well for secondary mains, sidings and yard tracks if there are good ties and roadbed under it.

100 lb. rail lasted a long time under some pretty heavy traffic. While not the best rail for heavy mainline use today it will do the job.

110 and 112 lb. rail have carried a lot of mainline traffic over the years. I think the biggest complaint was the short (4 hole) bars that caused joints to bend.

115 lb. RE rail was/is great rail. Use it under anything without worries. The long joint bars (6 hole) keep the joints in good condition especially if supported by good ties that are kept tamped up. A little grinding on the ends (when installed!) and welding as needed will keep the track in great shape. I see unit coal and grain trains gliding over 115 lb. ribbon rail at track speed every day.

132, 136 and 141 lb. rail and larger are the newer standards for heavy mainline. I believe the main advantage is that the extra steel in the head will last longer as the rail wears in curves. The larger size rail will always compensate for poor tie and roadbed condition to a certain extent. But I don't think overly heavy rail is a good substitute for ties and surfacing.

It is my opinion that well maintained jointed rail of a decent size is just as smooth as welded and is probably safer. Its a question joint failures vs. sun kinks. I'm not sure of the research on this though. Welded rail is much cheaper to install and maintain and that is the reason railroads switched to it. There was a huge cost in maintaining jointed rail to a high standard and few railroads did this in the days of financial instability in the '60's and '70's on many lines.