Mainly because they have priced themselves out of the market and people are too lazy to walk to the buffet when there is a trolley coming through. Cut the price of tea and coffee to around £1 while halving the size of cups, and you would see a massive upsurge in buffet customers.
Besides, there is hardly the time to get there and back on a Bristol service.
Do FGW provide a Trolley service on their HST trains?
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Are there any Mark 3 (Non-Sleeper) carriages that are unused?
Also, could they replace the Class 90/Mark 3 stock on the GEML and tranfer the Class 90s to FOC's and the Mark 3s (after converting them to HST standard) to FGW?