The first place I looked was in the "Alpines" section of the garden centre. The plants in this section are generally small and slow-growing, so they shouldn't dwarf the trains, and shouldn't require too much maintenance. One of the early finds is described below.
Arenaria balearica is a low, spreading plant with tiny leaves. It flowers on short stems in early summer, and retains its leaves all year. It likes well-drained soil, sun or part shade, and is drought-resistant. It is ideal for providing ground cover close to ground-level trackwork, reducing the chance of earth splashing over the rails when it rains.
I have just found this post sitting waiting patiently as a "draft post". Although it is a bit shorter than originally intended, it might be useful to someone, so her it is! Perhaps I will continue the topic in a later post.