Although I have not come across specific info, photographs lead me to believe that there were dedicated trains of flat cars assigned to certain factories producing tanks and other military vehicles.
Having to block these loads every time would have been time consuming and a waste of wood material. If the blocking were in place, loading new tanks for shipping would go a lot quicker. In March of 1943 alone, Chrysler produced an astounding 678 M4A4 Shermans. Dedicated trains of flat cars in arsenal service only makes sense.
A Sherman is being lowered onto a 70 ton flat car that has blocking in place, implying captive service. I believe this is a late model small hatch Ford M4A3 75mm (circa August-September 1943).
But not all Shermans had the same wheelbase. The M4, M4A1, M4A2, and M4A3 all could be loaded onto a flat car without altering the blocking.
However, the Chrysler M4A4 (and the rare 75-only M4A6) had a longer hull to accommodate larger power plants. As result of this the suspension bogies were spaced farther apart and the wheelbase increased. Therefore, the Chrysler Tank Arsenal must have had dedicated trains to deliver their unique Sherman model. Below, early Chrysler M4A4 Shermans on flat cars (note the increased spacing between the bogies at the road wheels and mixture of flat car types).
At a light weight of 65,400 lbs (32.7 tons, M4A4 pictured above), two Shermans could be loaded onto 70-ton flat cars. The Army preferred loading two tanks per car, but many more older 50-ton flat cars were available where they could only be loaded singly.
How the loads were blocked and secured can be studied from period photos. These can be found online at various places and on the Sherman Minutia website (http://the.shadock.free.fr/sherman_minutia/index.html).
RAILWAY PROTOTYPE CYCLOPEDIA Volume 20 has a section on military loads and how they were blocked, as well as many other types of flat car loads. At one time this was a difficult volume to get due to all the interesting flat car loads, but it has been reprinted and can be found online at the time of writing.
The blocking was standardized but there are variations…
Early M4A2 Shermans (July 1942) at Pullman-Standard plant in Hammond IN on C&O flat car. The tie-downs are not common.
Early M4 – no tie-downs but with blocking underneath which I believe is not common. Note improvised gun crutch, also not often provided.
Early M4A1 on GTW flat car – note unusual tie-down through bogies over tracks. Note posts in stake pockets -this is common.
Another M4A1 on Canadian National flat car – CN cars used in this service on U.S. roads. No tie-downs.
Ford-built M4A3 Shermans with blocking much smaller than typical and no posts in stake pockets. Unit on move.
Light tanks had similar blocking, but no posts in stake pockets. M5 Stuart on PRR FM flat car.
M3A1 halftrack on CRI&P flat car with wire tie-downs. The blocking is typical for wheeled/light armour vehicles. This is a unit on the move as opposed to a loading from the factory.
Wheeled vehicles (Jeep and cargo truck) typical blocking. Note wire tie-downs. Wire was run through stake pocket up into a wheel opening and the ends tied together. Tension was applied by inserting a short smaller piece of lumber or stick in between the wires and turned turnbuckle-style until tight.
The fastidious modeller may want to apply the wire, but I find it is way too fine for HO scale and I do not.