There where no real road network in the Soviet union. The majority of transportation was made by trains.
You are overestimating the value of tank truck transportation that would be almost impossible in muddy road of Soviet union.
Soviet union units move for hundreds of kilometers thru Europe and far east with little truck support.
Panther was a far superior tank than the Sherman weather it could be loaded on truck or not is rather irrelevant.
the infrastructure in the soviet union were pretty bad by western Europe standard, both rails and roads alike. Neither system were extensive by any means.
In practical terms you can't rely on rail entirely, nor would you want to fight so close to a front line supply hub. Supplies are usually carried extreme long distance by rail and rest of the way by trucks (or horse).
Rail lines are also known and limited geographical feature. Basically everyone know where every rail line is, thus further limiting the venue of advance and make your attack more predictable. (road also have this problem but to a significantly less degree).
and logistical consideration for a tank is never irrelevant. Having the best tank means nothing if you can't get it to the front.
The heavy axis tank were stretching/breaking the practical engineer technology of the time. The panther was significantly less reliable than the sherman and the tiger even worst. Forcing them to drive itself is just exacerbating the problem.
Tanks are definitely heavier on average nowadays, but they are also supported by better engine and transmission technology since. Medium tanks in ww2 hover around the 30 ton range for a reason: It was basically what the limit of what engine and transmission could reliably support in those days.
I would definitely prefer to be in a panther over the sherman in a fight, but it is the harsh reality that the weaker sherman is more suited to logistic and thus winning a war.