[1][2] The crash arose from misunderstandings between staff which effectively over-rode the safe operation of the Electric Train Tablet system protecting the single line.

The Cambrian Railways, which traversed Wales from Whitchurch in Shropshire to Aberystwyth and Barmouth, via Dovey Junction, contained a number of single line sections.

Shortly before midday on 26 January 1921, a west-bound stopping train from Whitchurch and an east-bound express from Aberystwyth were approaching Abermule from opposite directions, and were due to cross there.

The other three station staff at Abermule were Signalman Jones, Porter Rogers, who was seventeen, and a trainee booking clerk named Thompson, who was only fifteen years old.

He then checked that the express was running to time, and was informed that it had just passed Moat Lane Junction on the far side of Newtown, as scheduled.

Before Porter Rogers could call to Signalman Jones to release the ground frame lock, the stopping train arrived.

Thompson also mistakenly told Lewis that the express was still "about Moat Lane", presumably from having overheard some of Jones's earlier telephone conversation.

The express was travelling at about 50 miles per hour (80 km/h) and the crew were about to begin slowing for the arrival at Abermule, when they saw the stopping train heading for them on the same track.

Fifteen passengers, including a director of the railway, Lord Herbert Vane-Tempest, and the driver and fireman of the stopping train, were killed in the collision.

The Inspector (George) took the Montgomery-Abermule tablet on foot to Abermule station and replaced it in the correct machine in the presence of witnesses, to allow a breakdown train from Oswestry to reach the scene of the crash and assist with freeing the injured.

The slack working practices had been allowed to develop over several years by Stationmaster Parry and Signalman Jones.

Finally, the awkward layout of tablet instruments, signals and points levers at Abermule meant that it was possible for conflicting movements to be made.

However, this would involve the rebuilding of the station buildings and signal boxes at Abermule and several other small stations, and it was suggested that alternate single-line sections use the electric tablet system and the older electric staff system; there would be no possibility of mistaking a staff for a tablet.

Although both trains stopped 312 m (1,024 ft) apart before any collision occurred, a subsequent safety investigation cited the Abermule accident as having "clear parallels with the events leading up to the incident at Romney Sands".