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Is there any particular reason this train routes via Melksham and Swindon - I'm guessing it's to retain route knowledge for diversions, given it doesn't actually stop anywhere between Taunton & Reading?
Between Christmas and Easter, the track layout at Reading only allows trains from Westbury to access platforms 7 and 8 (and possibly 9?). So I think this train has to go via Melksham so that it can have a platform at Reading (booked in platform 11).
Once the Feeder lines open at Easter, trains from Westbury should be able to access all platforms (possibly excluding 9, I've forgotten).