Most train services through Melksham are run with a single coach unit of class 153. The early morning (Monday to Friday) train ia two car 150 train as the 153 wouldn't have enough capacity on its return trip from Swindon to Southampton on some sections of the route.  Weekend trains are also class 153 or 150.

These trains are all about 20 years old. Both the 150 and the 153 are "pacer" trains - the 153 originally being 2 car class 155 units that were divided up and re-engineered to provide a small driving cab at what had previously been the inner end. The 150 and 153 units can work in multiple, and I have seen a 4 car formation on one of our services through Melksham.

It'll be interesting to see what's used in the future - how much longer we'll have the 150 and 153 units.  According to "Passenger Railway Planning" - ISBN 0-86317-174-5, train reliability can be graphed like a bathtub - the "bathtub curve".  After a series of teething issues, the train will run reliably until it's about 20 years old after  which it's getting a bit long in the tooth and starts giving reliability problems.