More on this from FOSBR :

It appears that the incident was due to the presence of a broken down train somewhere beyond Clifton Down, and possibly another train sent out to recover it. It is therefore entirely the responsiblity of First Great Western (FGW) .

Despite this situation being already known, it appears that FGW chose to allow passengers to board at Clifton Down, and then moved the train forward the short distance to the red signal, where it stopped, effectively confining them just beyond Clifton Down for nearly an hour, which could have been predicted before leaving the platform.

It also appears that it was only possible to detrain because the service was unusually made up of three coaches, leaving the last door still on the level part of the platform. Apparently FGW have no procedure for setting back in these circumstances, and without the fortunate accident of an unusually long train, it appears likely that the passengers would still be confined.

Two replacement buses arrived about 15 min after the passengers were disembarked, which is hardly prompt given that the problems must have been known probably about two hours by then.

The whole affair, arising first from reliability problems entirely within FGW responsibility, appears to have been grossly mismanaged, and as slowly as possible.