In pre-Beeching days, the route through Melksham was quite busy, with express services between Trowbridge and Paddington being a big draw. However, in effect most traffic would be to or from Bristol, and Melksham has no direct link with Bristol courtesy of being on the eastern side of a triangle between Bath, Trowbridge and Chippenham. In addition, there are bus links between it and all of the places mentioned previously, giving it adequate (if no better) links to local stations around the area, especially Bath.
However, FGW are said to be interested in restoring an improved sservice, and as such it has got a slightly better outlook than before. But with no easy link to Bristol, barring a change or bus (by which time you may as well get a second bus from Bath; they're frequent enough!), it is not likely to happen any time soon. This is compounded by the route being single track only.
If you were wondering, most trains run as extensions of the Swindon to Cheltenham local service, which runs alternately with HSTs to London, to and from Southampton.
The best way to claim that bit of track, without resorting to using the near-Parly service that Melksham gets, is to wait for a Westbury/Taunton service to be diverted that way, which does happen sometimes - or for GOCO, as Anthony says.
