The train proceeded 600 yards (550 m) into the tunnel but then began to slip backwards, as shown by skid marks observed afterwards on the rails.
The railway company had anticipated this possibility on the very steep incline, and had a rule that the track should be left clear behind unassisted trains until an indicator showed that they had reached the top end of the tunnel.
However, the signalman had allowed an empty local train to Alloa to move forward into the tunnel in order to reverse into its correct platform.
However, when tested after the accident, the right-hand sander was found not to be working as its steam supply line had broken without warning.
Col. Trench placed the main blame for the accident on the signalman for not following the rule designed to give a safeguard in precisely this situation.
A more spectacular accident occurred on Saturday 12 August 1911 when the 1.5 pm express from Edinburgh failed to stop; the engine demolished the buffer stops, carved through the concrete circulating area (narrowly missing the Lord Provost) and just penetrated the refreshment room wall before coming to rest.