Werner, if you are willing to test something....
The floor of you LocoShed.... I would pour a watery mixture of creatstone on the outside of the track level with the top of the rail.
For the inside I would fill it till the top of the sleepers. Once it is dry you can sand it lightly and then paint. Should work well and will look GREAT.....

If you do not want it that permanent, build it from bluefoam, easy to work with, to shape and sand. Same way I built my HO platforms.
I would also build the outside of the track level with the top of the rails and inside level with the top of the sleepers. Should work quite well.
Just my idea.....
Cheers, Evert.