There are going to be difference and additions to the below, but core routes:
Westbury - Weymouth via Frome, Castle Cary & Yeovil.
Westbury - Portsmouth Hbr
Westbury - Bristol Temple Meads via Bath
Westbury - Swindon via Melksham & Chippenham
You will also sign diversionary routes.
Other routes you may also sign
Swindon - Cheltenham Spa via Gloucester & Stroud
Bristol Temple Meads - Gloucester via Bristol Parkway
Bristol Temple Meads - Cardiff Central via Patchway
It will also depend which link you are placed in, as routes vary