There’s a rickety iron bridge in Indy that was too weak to haul fully loaded covered hoppers over,so they were partly unloaded and the excess trucked around,of course we had 4 axle locomotives which had greater loading per axle than those hoppers! We would pray before we crossed. Some clever railroads would put a spacer car between the engines and run long jumper cables across a flat car or through a coach to spread the weight out across the bridge. Of course proper bridge maintenance would solve this issue!
On my railroad we use 4 axle power on sub-standard tracks, like some of our industrial leads. Six-axle powers are much harder on track and liable to derail in a curve, when the ties are rotten.