Why is the route through Stroud so slow compared to the long way round via Bristol Parkway?
1. 75 m.p.h. stock schedules (class 150 / 153)
2. Twists and turns and climbs on the route
3. Services reverse at Gloucester on their way to Cheltenham Spa
And the number of stops is the other thing. If you break it down, then (approximating a bit):
Via
BPW» it is 63 miles with no intervening stops, and takes 55 minutes in the two fast trains. The linespeed is mostly 90-100 mph, and the time reflects that plus the single stop and start for each train.
"Direct", it is 43 miles without the Gloucester detour, and three stops plus Gloucester on the way. It takes 74 minutes. The linespeed is now mostly 90 mph or more, but the train only does 75 mph. Going via Gloucester adds 17 minutes, which looks rather generous. 43 miles with no stops would take about 43 minutes, at best. And the three other stops could plausibly add the other 14 minutes.
Upgrading to 90 mph 165/166s trims about 4 minutes in running time, and perhaps a bit more in better acceleration from stops (especially if any of them are uphill). Doing something about Gloucester has more potential, even without the through platforms that are proposed in the Route Study.
But the main message is that the short route is "only" 20 miles shorter, say 20 minutes less, and being a slow stopping train adds back twice that much.
