A signal (A491) in the tunnel between Stratford and Leyton had been damaged, and this and the preceding signal (A489) were showing a permanent red aspect. Trains were being worked slowly past the failed signals under the "Stop and Proceed" rule, under which trains should proceed with extreme caution, typically less than 10 mph (16 km/h). However, one train collided with the back of another which was waiting at signal A491, and the first and second coaches of the colliding train were partially telescoped.
The Inspecting Officer considered that the extent of the damage suggested the speed was in the region of 15–20 mph (24–32 km/h), and when the driver had passed signal A489, he had simply coasted down the steep down gradient, not expecting to find another train before the next signal. The driver claimed to have been travelling slowly and that his vision had been obscured by a cloud of dust, but it was felt his memory could have been affected by concussion.