train polytunnel deepbeds

The other day on the train my brain start to wonder and wander. There is a funny use of space where the roof dips down and it seemed to be the right dimensions for making a vegetable bed on the roof. Then there is the waste of heat in fairly empty train carriages. And heat rises doesn’t it? So would the heat rise to give warmth to the bottom of my vegetable beds. We could stick a polytunnel with hinges over the deep beds on the train roof. Would a train in motion disturb rocket, mizuna and spring while they tried to grow?Would they not like the motion or light levels?

Then could the night soil from the toilets on the train be used to make compost for the vegetables?

I was wondering on the way home how many vegetables and herbs (if any) could be grown on the roof deep bed system of the 10.35 Lambeg to Lisburn train?