While it's a tempting scenario for rail advocates to envision, I'd guess that a vigorous campaign would be waged against any attempt to roll back truck weight/size regulations from all corners including economic interests, and, quite possibly, environmental interests. Given that so much freight carried by truck will never be convertible to rail (given the tremendous conversion capital costs and rail inefficiency in much of trucking's domain) and that the bulk of freight traffic that would potentially shift to rail would have to be intermodal in nature with trucking involved, the impact would be substantial. Even on the safety front, I think the case could be made that a redux in truck size/weight would lead to more trucks/trips on the road which would lead to more accidents, at least in those places that would continue to host truck density.
Perhaps, 2nd Trick, you're speaking from a broader view and not simply about a regulatory roll-back, but, instead with the other government lever of promoting modal diversion by plowing money into the capital improvements necessary to build railroad capacity, be it for overhead clearances or expansion of linehaul or terminal infrastructure. I'd agree that these promotional initiatives are probably where we'd continue to see public policy focused, but, also with concurrent government promotion of safety, fuel economy, environmental, and, quite possibly, highway congestion improvements with the trucking industry. And, then,too, it doesn't seem that the path of government investment in rail freight capacity is completely assured given political and rail industry dynamics, limitations of rail vs. trucking, or with the potential technological/systems advances in trucking or distribution that could blunt a meaningful modal shift.