The RAIB said both trains involved were fitted with wheel slide protection systems, a similar function to anti-lock braking systems on road vehicles.
The Cambrian line is equipped with the European Rail Traffic Management System, a form of railway signalling which transmits signalling and control data directly to the train.
The railway approaching Talerddig from each direction consists of a single track, and to allow trains to pass each other there, a loop is provided.
The loop has points at each end which allow trains to enter a short length of track adjacent to the single line.
Westbound trains climb an ascending gradient to enter the loop and, on exiting, rejoin the single track as it descends towards Llanbrynmair and Machynlleth.
During the preliminary examination, RAIB said it found that the westbound train had been due to stop in the loop at Talerddig to allow eastbound train to pass.
Initial analysis of data from the on-train data recorder (OTDR) fitted to westbound train shows that the driver applied service braking to slow the train as it neared the loop at Talerddig.
Around 40 seconds after the first service brake application, the OTDR records an emergency brake demand being made and it remained in place until the collision.
OTDR data shows that wheel slide started during the service braking and was constant during emergency braking.
The westbound train then entered the loop at Talerddig. Although it slowed while passing through the loop, it did not stop before passing the block marker positioned near the exit.
The train subsequently exited the loop, rejoined the single line, and continued to travel for around 900 metres on the descending gradient, before colliding with the eastbound train.
