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.
Elfin thyme
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)
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