Are these events Battles of the English Civil War?
If so is 5 Oxford, where the city was besieged three times.
Well, my initial thought was civil war sieges, but there aren't enough - and there's the second type of event too. But if that's battles, there's too many, even in the
GWR▸ area. But the real problem with fitting those to the clues is the vagueness of the concepts of a railway line going near or through the site of one, given that mostly they don't go through the historic walled part of a town anyway.
Plus of course who's counting - Taunton, Basing House and Worcester have a better claim to three sieges - and you may not even need to count Worcester as two sieges and one battle, since the Battle of Worcester was in effect a very very short siege.
But, prompted to have another look, I think it sort of fits like this:
1. Newbury - two battles
2. Reading - one siege
3. Oxford - not sure why it counts as "last"
4. Stow-on-the-Wold - one battle
5. Worcester - two/three sieges
6. Gloucester - one siege
7. Bristol - two sieges, though one was little more than a day
8. Taunton - usually described as three short sieges
9. Plymouth - beiseged for all of the first civil war