I imagine most American states have their own limits on axle weight as in Europe. In the UK 44t is the heaviest you can run without additional permitting etc.
It will just be an additional constraint, there are lots of loads which are limited by cubic volume rather than weight. However, it wouldn't work well for say bulk tippers which are almost always at their weight limits (and transporting lots of low value products).
I'm not sure about the braking argument. Trucks spend most of their time cruising along. They also have exhaust brakes and retarders to avoid using the service brakes on hills for example.
But you are correct that a modal shift to rail makes a lot of sense.