Wright was responsible for the aerial shots of the train from a hired aircraft, while Watt filmed the interior and location shots. The men in the sorting coach were real Post Office workers, but filmed in a reconstructed set built at the GPO's studio in Blackheath. They were instructed to sway from side to side to recreate the motion of the moving train which was accomplished by following the movement of a suspended piece of string. Early attempts to simulate motion was done by shaking the set, but Watt wrote: "It just rattled like a sideboard in a junction town". Attempts to film while shaking the camera also failed as it merely produced a wobbly shot. Among Auden's first tasks as assistant director was taking charge of the second camera unit as they shot mail bags being transported at London's Broad Street station, where a replica postal train was assembled for a night so the crew could film and record sound to match the footage they had captured at Crewe. Wright later thought it was "one of the most beautifully organised shots" of the entire film. The filming session began at 5:00 pm and lasted for 14 hours without a break. It included filming of the wheeltapper, who became the subject of an inside joke at the film studio when the crew forgot to shoot his scenes until the next morning. They adopted the term when the unit had missed a shot. Shots taken at Broad Street were incorporated into the Crewe sequence in the film.