Is it the Take Along Thomas, the wooden Thomas, or the regular plastic Thomas? All the other trains in the particular set would be compatible. If it is the wooden ones then most generic ones will work with it too. Almost all the wooden ones work together. Look for ones that have the same shape track.
Like the first poster said, if it's the wooden Thomas track, almost all the generic, off-brand trains will work with it. My son has a Circo brand train table/train track and all the Thomas trains fit on it.