"SkyFreight": Freight on SkyTrain? Why not?

I've been wondering if there has ever been any consideration to using SkyTrain for local shipment of goods/freight. It seems like a perfect way to increase the use of a system that is already in place, and being fully automated, would be easy to integrate into it. It would also be an additional revenue source with relatively small start-up costs since the rail lines are already there. Let's call it "SkyFreight".

"SkyFreight" cars could run independently in the gaps between passenger trains. There could be short branch lines or sections with a third track that would divert the freight lines to small depots beside the existing tracks where local trucks would pick-up and drop-off. A standardized container system would make it easy to onload and offload, and should be designed with long-distance rail in mind, so goods could be easily transferred to and from heavy rail freight trains (CN, CP).

Being able to take goods from depots on the edge of the city into the centre by "SkyFreight" would drastically reduce the number of transport trucks in the city. I can see this being a great way to ship garbage and recycling out of the inner city to a location much closer to the disposal facilities, eliminating thousands of garbage truck trips. This system would also integrate perfectly at the airport - imagine if containers could come off a plane, be rolled onto the "SkyFreight" car and be on its way without sitting in a depot. Courier services like Fedex and UPS could see a huge benefit from such a system, as they could locate depots next to train lines (in the most populated areas) and ship containers of parcels from there directly to the plane at the airport, rather than all their trucks having to drive there. Trains could run uninterupted at night when the passenger trains aren't running.

It's a very futuristic idea - automated "SkyFreight" cars (of a distinctive colour and design to differentiate from SkyTrain cars), continually shuttling goods around the lower mainland with a minimum of human input and releasing no pollutants into the air. Makes me think of scenes only seen in movies of cities of the future.