The NYC J-3A Hudsons were built new with nickel steel boilers to save weight. After a few years, they exhibited "trans crystalline" cracks that was traced to the boiler water treatment used. Many of these engines had one or more boiler courses replaced, and at least five and possibly ten super Hudsons received welded boilers. A batch of ATSF 4-8-4's built at the same time also had this problem, and I believe that these boilers were replaced.
The boilers of all of the Niagaras were replaced with welded boilers in the 1948-49 time frame. Due to their high availability and utilization, the RR might have thought that this was in line with normal boiler life for engines that as a fleet were running over 18,000 miles per month per engine. The last Niagara to have its boiler replaced was #6015, and the boiler used for that replacement was the boiler from poppet valve Niagara #5500, which had a welded boiler. The #6015 was the last Niagara to receive class repairs at Beech Grove. The above info is from the records of the last Chief Mechanical Engineer of the NYC, William Edson.
I was not aware of any state or federal restrictions re use of locomotives with nickel steel boilers, and would appreciate the original source of that information.
