The Accesrail website states:
First Great Western now provides, thanks to AccesRail, its first air-rail intermodal solution. This allows passengers to go to/from Heathrow or Gatwick Airport to their First Great Western destination of choice.
Passengers receive an intermodal ticket that covers both their flight and rail journey. Depending on which airport the passenger travels, train connections are slightly different.
If I have an intermodal ticket, does that mean I am guaranteed a connection from the train to the flight and vice versa? I mean in case of delays. At one point the Swedish railways SJ and the Swedish airline SAS had such an agreement (perhaps they still do) when connecting from the train to a SAS flight within Europe.