Presumably one major problem is line capacity ...
It so much depends on who you talk to, how you measure capacity, how capacity on one element interacts with capacity on another, and how much allowance you have in capacity for events off-plan - i.e. trains delayed and needing to be dealt with somehow.
Network Rail will tell a story.
GWR▸ will tell a story that's not necessarily the same. Go-op will give you a story, as no doubt will Grand Union Trains. Freight operators will ave their views, and then it will all to to the
ORR» to adjudicate. Then the
DfT» will instruct franchised
TOC▸ (s) to run a particular service anyway, with operational elements not being the only, or even overriding, ones to be considered.
There is - probably - room to run an hourly Swindon to Oxford, and that would be made more robust and also useful were it to reverse at Didcot at first. The space gets sucked up by super fast paths and by paths needed by the new stone quarry freight operator is some come this way, and by
HS2▸ construction traffic to / from Westbury and Quidhampton. You can make some savings by joining a Bristol Superfast to a South Wales super fast - 5 cars each - at Bristol Parkway; these paths will be expensive ones not calling at either Swindon or Didcot and if even one can be saved.
Now - up from Didcot to Oxford - I wonder how reliable the timings of services, especially freights and cross countries - are ... and how that looks also with that HS2 traffic mentioned above. That is, of course, making assumptions about HS2.