From what I was reading the axle loads on some steamers was not that light , I didn't notice their lengths but I suspect it may be more than 21-22m .
Some of their quoted max speeds looked interesting and I think one was quoted at a little over 90 miles per hour . Hmm , 22TAL at 145 km/h with hardly modern suspension .
Bridge wise they support the total mass of the loco/s with the rails and sleepers distributing this load . Todays heavier rails and sleepers should be better than what was around 40-50 years ago .
For the steamies , what infrastructure limited the areas D57s could go ? I think it was Junee south and Lithgow west , can't remember north and Illa .
The 57 class were allowed to Thirroul on the Illawarra but were not permitted on the North at all since a number of bridges were too weak (Cockle Creek, Ourimbah, Wyong, all of which were replaced in the early 1960s) but significantly because they were too wide for some tunnels.
The weight of the D 57 was understated on the diagram and it was significantly heavier, notably on the trailing truck.
But they were initially limited to Harden on the south and were only allowed to Junee after the duplication in 1946. I'd guess they were allowed to Cootamundra before WWII. The 57 class locomotive weighed 141 tonnes carried on seven axles, with significantly less on the leading bogie than the driving axles, which reduced curve forces. The locomotive and tender were 26.6 metres long.
The AD60 Garratt did weigh more than 260 tonnes but they were 33 metres long with an axle load from 16 to 18 tonnes. These were designed for branch lines and the whole loco would rarely be on a single bridge span at once.
One bridge that couldn't take a 180 tonne locomotive would be the Murray River bridge which is now single track since trains were not allowed to pass on the bridge in dual gauge days.
The heaviest locomotives allowed south of Junee were the 38 class. There were only 30 38 class and they almost never double headed in regular service but there are 120 (well nearly) NRs and they are usually at least double headed. Combined with much heavier freight wagon loadings, the same rail that has been there since the 1940s is getting a much harder time now with 134 tonne locomotives than it had with steam, even if the sleepers are new.