I mean, it's a bottleneck heading back towards London too, between the barrow roundabout and Stonehenge is often a mess in both directions. Mostly due to people looking at Stonehenge instead of the road.
It's less about improving the trains and more about making them cheaper! (Not that I would complain about some new lines either!)
If they took it right through the hills and brought out out by Winterbourne Stoke then it would be much better I think?
Most of the traffic is through traffic anyway. Bringing it up by Long Barrow Roundabout (it's in the name!) was crazy.