Recycling old subway cars

as far as i know, the majority of subway cars are sent to the junk heap, or are demolished for scrap metal. This is the only other thing i could find on it http://chase.inthebasement.us/2004/08/recycling-subway-cars?cat=7

a few weeks ago on a train ride from Montreal to Toronto, i came up with this idea. Essentially, subway cars would have the tops cut off of them. They would then be filled completely with rich soil. Trees, shrubs and all manner of flowers, hopefully wild, would be planted on them, creating a
portable, urban green space from an old subway car. They could be put in parking lots, town (city) squares, or even left on the subway line near stops- anywhere that needs a bit of green. Because of their height, i believe ( sorry, don’t know much about gardening) that even trees could grow in them. They would have a walkway down the middle, and stairways going up to the garden where the doors would be. This would work well because of the guards on the sides of subways doors, which stairs could be built right into. Just looking for comments, helpful advice, ideas. I’m only in grade 10, so please don’t be to harsh. The drawing is just a rough sketch to give you an idea.

p.s. i just realized i forgot to draw wheels

I think the idea is very intersting, but some issues i forsee is that the metal would rust over time, and after a certain period, it wouldn’t become as beautiful as when it was created.

The other issue is roots. its like having plants in a pot, as the plant grows bigger, the pot needs to be changed, because the roots wil have no space and will probably break through the metal.

also, i would think that having a proper recycling process to reuse all of the metal, plastic, rubber and fabric that makes the wagon.