Some of the speed restrictions may be due to the pounding that steam puts on the rails. Maybe modern road beds are not built to hold up under it.
Modern steam locomotives like the 765 or 611 do not pound the rails. Many railroad crews that have ridden aboard the 765 have commented that it rides better than most GEs. Some guys on the 765 crew claim the loco rides better than our tool car.
Secondly, main line track today is much stronger than the right of ways of the 40s and 50s. Todays main lines are mostly welded rail and built to handle 315,000 pound freight cars. A "heavy" car in the steam era was HALF that weight.