there is some kind of garden mini-gravel that makes great balast. of course over time it just sinks out of sight. something the railroads sometimes do when they have a problem with that is to use something called geo-textiles (which is basically just giant wide felt on really long rolls) under the ballast. when i had my garden, i used plastic sheeting, cut to where it was the same shape as the track, cut to almost no more then the same width. this didn't work perfectly because some of the plants just grew around it and took over anyway. but it the ballast always looked good when i first put it down. i also used no other physical understructure then to compact the earth under where the track was going to go with a board and a hammer.