Off-duty Pc dragged by suspects' car.
Pc Terry Wright was only freed when the car window clamping his arm smashed and he was thrown to the ground and knocked unconscious.
He was recovering at the home of relatives yesterday after being treated in hospital for cuts and bruises.
Police have launched a hunt for a man and woman spotted in the car involved in the incident.
Pc Wright, a 43-year-old British Transport Police officer, became suspicious on his way into work when he saw the pair, who appeared to be rolling a cannabis joint while parked outside Gloucester railway station.
He revealed he was a policeman before reaching into the open car window to grab the ignition keys as the driver went to start the car.
It shot forward, dragging Pc Wright with it before the window smashed and he broke free.
The couple drove off toward Gloucester town centre.