At a little after noon, "Ladas" with the two carriages started back down the mountain. About above Clogwyn, where the line runs on a shelf formed across a steep drop, the locomotive jumped off the rack rail, losing all braking force and accelerating down the track. At first it remained on the running rail and the driver tried to apply the handbrake, but with no effect. Realising that the train was out of control, the driver and his fireman jumped from the footplate.