Two postal workers were pistol-whipped and other staff threatened at gunpoint during the raid on Friday night.
The documents were stolen in mail bags, but it may be weeks before the exact contents are known. Blank tax discs and postal orders fetch high prices on the black market.
At about 7pm, after the train pulled in, the robbers drove into the station in a blue Ford Transit van through gates in Eversholt Street. A padlock on them had been cut in advance.
Six men, two in Post Office uniforms, grabbed a train driver and took him to a mail coach, where he and other rail and Post Office staff were forced to lie on the floor, some with their hands tied behind their backs. The raiders escaped through the same gates with a large number of mail bags and disappeared into heavy traffic.
Two postal workers were released after hospital treatment for minor injuries. British Rail has offered a pounds 10,000 reward for information leading to the arrest and conviction of the robbers. BR said platform one, where the robbery happened, was almost exclusively used by mail trains. The train that was robbed had arrived from the north.
The police said that, although members of the public must have been at the station at the time, the raid was carried out so professionally and quickly it passed largely unnoticed.
John Prescott, Labour's transport spokesman, last night called on the Department of Transport to launch an immediate investigation into the raid and the effectiveness of security at the station.Reuse content