He's looking for Trains2004 and either
Good point, I forgot to re-read the first post
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A 159, you've got no option other than the DLS atm. There are FGW 3 car 158s
here, but they require assets from the DLS.
Not sure what ran on the Somerset and Dorset.
Tbh, I'd recommend upgrading to TC3, which I think has at least some of those steam locos built in (As well as the wonderful Mk1s), and they're really good models that you can't get in any other version.
The TC3 site is
here. Have a look around it, at the built in content and the updates (There's an update every friday which is free, be it coaches, locos, signals).
You're limited with backwards compatibility a bit, but, because you've got TRS04, you can probably import a lot of the built in content to TC3 to get stuff to work in it that wouldn't otherwise.