Thymes are perennial sun lovers. Lots of varieties like lemon, english and wooly will work well. Scotch/Irish moss will take a bit more shade but likes it moist. There are many dwarf sedums that are useful on a garden railway (they do better in sun). Creeping Jenny (Lysimachia nummularia) is another choice (sun/shade).
Plantings will vary with you climate. You didn't post your location.
As noted, thyme is very good. The smallest of the thymes is "elfin" thyme. This grows slowly in a very dense mat..., so dense weeds can't grow through it. When it gets to your track, it can stop a train. Best way to control it around the track is to plunge a putty knife into it parallel to the rails cutting through the roots. You can then treat the area along the track with Round-up without killing everything.
Creeping thyme is also nice, but larger scale and more profuse.
If you have shade and lots of water, baby tears are a great ground cover. I also really like Corcican mint, chamomille, and dimondia margarente.
Dimondia is drought tolerante and can be walked on without harm (around the volcano)
Posted By Ray Dunakin on 11/14/2008 10:35 PM
Todd, what's the origin of your railroad's name?
We have a pair of endangered California Desert Tortoises. I actually have permits to keep them. We acquired the female at the same time I built the railroad. We had already had the male for ~5 years. The lizards think I built it for them. They live in the buildins and under the turn-out covers. They are very tame and will come up to me in the garden for hand outs. If I find grasshoppers, I fing them on the ground and when stunned, the lizards get a meal. I've actually been trimming trees, and had a lizard climb my pant leg to eat a bug on my pants.
This link takes you to the chronology and opening day, 1997. George Schreyer came over and documented these operations. Our first run was after just 6 month of construction (all on my own with all rails having soldered jumpered). It took me a month just to build the control panel. I also build my environmental consulting business during this period. I do air quality and noise analyses for Environmental Impact Reports.