Trucks with tarps often had the tarps removed and stowed in the bed. These tarps, when up, were secured with ropes tied to metal loops on the sides and ends of the cargo body. Due to the wind generated by a fast moving train, these ropes would work themselves loose and tarps were known to go flying off.
Here is GMC CCKW 2-1/2 ton 6×6 cargo truck without tarp and a jeep load. Note blocking – on tandem axle vehicles, the wedges are on the outside of the pair of wheels only.
But, these trucks are seen with tarps on and hopefully well secured. Note ropes and trailer cargo load. Jeeps have tarps taken down and stowed. The half track on the left has the tarp on – this is a different situation as the tarp was factory made with straps and buckles to fasten securely into loops riveted on the body side armour. Tarps were desirable because they kept the weather out from the inside of open-topped vehicles.
In the early part of the war military flat car loads were often seen tarp’d-down. This was likely a security measure. Below, covered M3 Lee Mediums on the B&A. I believe once the Lend-Lease Act was passed, production ramped up dramatically and tarps were no longer used.
These bound-for-Britain Pullman-Standard M3 Grant Mediums look as though they have fitted covers – I think most uncommon. This may have been a British requirement as the Grants were prior to Lend-Lease and therefore were directly paid for by U.K. contracts. I believe the British policy was that every tank should have a tarp while this was not so for the U.S. Army.