The off-duty officer had seen a saloon car on the crossing and then watched as the barriers moved into the prevent-entry position with the vehicle still there.
The Thames Valley Police officer went to the emergency phone at the crossing to summon help - but the London to Plymouth First Great Western express train struck the car before he could get through.
The male car driver was one of six people killed in Saturday night’s accident at the automatic half barrier outside the village of Ufton Nervet near Reading. Some 150 of the 300 passengers were hurt, with 15 people still in hospital.
As rail unions and safety experts called for a review of level crossing policy, Transport Secretary Alistair Darling promised a full investigation into the tragedy - the first at a level crossing to involve passenger deaths since the Lockington disaster in Yorkshire in 1986 when nine lives were lost.
As workers prepared to clear the tracks of the derailed eight-car train, Deputy Chief Constable Andy Trotter, of the British Transport Police, described the events involving the off-duty officer.
He said: “An off-duty Thames Valley police officer was driving along the lane. As he approached the crossing he saw a stationary saloon car on the crossing and he was obviously concerned about what was happening.
“He stopped his vehicle and before he could do anything the barriers came down.
“Realising there was a potential disaster he ran to the emergency phone to call the signal box, but before he got a response, the train came through.”
The officer was on the phone for a matter of seconds, said Mr Trotter.
Mr Trotter denied reports that the officer had talked to the driver, who was alone in the car.
The driver of the Great Western train, which had left London around 5.30pm, was among the dead.
Passengers told how they used mobile phones and novelty glowsticks as impromptu torches to guide them to safety after the collision.