Aren't metals pumped via a slurry? Which makes me ask why you loose all of your metals if you have no storage. Shouldn't there be some left in the tubes? Though I guess it wouldn't be much.
I believe that the help file says the metals are stored in a suspension of small metal particles and oil (or some other inert substance). Since it is a suspension, the tanks contain some sort of stirring apparatus that prevents the metal particles from settling out.
That's probably why the metals are lost when a storage facility is idled or destroyed. At idle there is no power to stir the contents of the tanks so the metal settles out preventing them from using it. If the facility is destroyed, obviously there is no more storage tank at all so the metals are lost.
I'm sure they can be pumped through the tubes but there wouldn't be a whole lot of metal slurry left in the tube if the storage facility is gone. (Consider what happens if you close the valve on a propane tank. There is still some gas left in the line but it will quickly bleed out, and any flames on the other end will quickly go out once the gas is used up).
As far as a monorail goes, I would think it totally unnecessary. You could say that it improves DIRT response time but the other argument would be, they have larger vehicles that move inside the tubes (I believe one of the mini stories in the help file involves a man driving a vehicle in the tube when the fuel storage facility at the spaceport explodes). They might also have special designated lanes or passages that only emergency/maintenance vehicles can use (like a lot of subways have).
