OK, I'll stick my neck out. I would be envisaging something like this as the ideal network for the area - and the kind of thing we should be aiming for in 15 or so year's time.
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In terms of investment, you're looking at 7 new stations, re-opening the southernmost bit of the old Cheltenham-Stratford line, and it would need some capacity enhancements (possibly grade separation and some 4-tracking) of the Bristol-Cheltenham line. Not the sort of thing that's going to happen without a step-change in Government support for the railways
But the point is, in this hypothetical network, I'm not slowing down long distance trains by giving them extra stops. If anything, I'm removing stops from them. But would that really be a problem when this kind of network makes it so easy to get to Cheltenham to change onto the long distance services?