Are there any airports which use a metro to transport cargo? Many airports around the world are increasingly being joined by Metro conveniencing people. Majorly, it is people who probably will use the metro. I do have a sneaking suspicion that the metro would also be a pretty inexpensive good carrier. Two things which I think work in its favor - 

a. Security on Airport Metros is higher so that much less hassle. 
b. Most of these Metros run on specific times so it's easier as it's consistent. 

Are there any airports that people know that transport air cargo via metros after being bought by Aircraft ?
DFW Airport's original train system had cars built to carry mail and luggage.  Most of the system was never used because it was slow, and there were worries that it was hard to train employees to interface with it. I would guess that a lot of airports have not tried such as system because their trans are optimized for passengers, so it is faster to just move luggage across the tarmac on trucks. 

Edit: Jonathan Walters mentioned that Denver also had a system, but it was buggy so they stopped using it.