I'm sorry but this "rule" makes no sense to me. First, not all military equipment is moved in solid trains. I've seen DODX flats with everything from Humvees to M1A1 tanks being moved as part of a regularly scheduled freight. Should you stop shooting pictures just because there are some military units in the manifest?
Even if it's a solid military movement, what information is being given to what enemy? Many of these movements are new pieces of equipment being delivered from manufacturers and an even greater number are simply deploying equipment from one base to another. If the military is concerned about not having the public see what's being moved, that's what canvas is for. Given the amount of money that's been spent developing vandal proof auto racks, I'm pretty sure the government could develop the same type of thing for military equipment if it was deemed important to national security.
Finally, what information is being given to the enemy we are fighting? Do you think insurgent fighters in Iraq or Afghanistan actually care about what's being moved here? If it shows up in one of those two countries, they'll try to destroy it but they have no massive amounts of military equipment they are moving around trying to fight pitched battles. This type of rule made sense in WWII, Korea, and even Vietnam, but it makes no sense today. This is another example of post 9-11 paranoia.