I quote from the official accident report:

The 5pm Mail Train from Milford ran into the rear of the 1.37pm Cattle Train from Carmarthen causing the locomotive to overide two cattle wagons and resting partially on the back of the third cattle wagon. The Driver and Fireman of the mail train were not injured but three Drovers and the Guard were killed on the cattle train plus five Drovers who were injured. The accident took place at Newnham, between Lydney and Bullo Pill on the South Wales Main Line via Gloucester.

The cattle train had stalled on the gradient due to bad rail adhesion. Both trains were working on a Time Interval System of approx 20min intervals but owing to the continued stalling of the cattle train the mail train caught up and collided with the train. It was recommended that a Block System of Telegraph should be introduced between Awre Jct and Bullo Pill plus other line's.

Photo: Rail Archive Stephenson