The interim report is now out (I can't find the original thread).
Quoting:
"RAIB’s preliminary examination has found that westbound train 1J25 had been due to stop in the loop at Talerddig to allow eastbound train 1S71 to pass. Initial analysis of data from the on-train data recorder (OTDR) fitted to train 1J25 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. This emergency brake demand remained in place until the collision. OTDR data shows that wheel slide started during service braking and was constant during emergency braking.
Train 1J25 then entered the loop at Talerddig. Although the train 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 train 1S71.
There is conflicting evidence relating to the speed of the trains at the point of collision. Initial analysis indicates that train 1J25 was travelling at between 24 km/h (15 mph) and 39 km/h (24 mph), while train 1S71 was travelling at around 10 km/h (6 mph) in the opposite direction. RAIB is continuing to analyse evidence relating to the collision speed, which remains an area of ongoing investigation.
Following the accident, RAIB undertook measurements of wheel/rail adhesion levels at various locations from the approach to Talerddig loop to the point of collision. These found levels of wheel/rail adhesion that were low.
An inspection of the automatic sanding system fitted to train 1J25 after the accident showed that the sanding hoses on the leading vehicle of this train (which would have been active at the time of the accident) were blocked and apparently unable to discharge sand."