An automated system that helps train wheels grip tracks failed on a train involved in a fatal crash in mid-Wales on 21 October, the Rail Accident Investigation Branch said.
One passenger died while four other people were seriously injured in the crash and a further 11 required hospital treatment.
David Tudor Evans, 66, had been travelling on the westbound service from Shrewsbury when it crashed into a stationary service near the village of Llanbrynmair near Powys.
The investigation found the train was fitted with a system which discharges sand automatically via hoses when it detects that the wheels are sliding during braking.
This is intended to increase friction between the wheels and the tracks.
The RAIB said an inspection of the train after the accident showed that the sanding hoses on the leading vehicle of the train were “blocked and apparently unable to discharge sand”.
Anthony Hurford, a passenger on the Shrewsbury-bound train, told the BBC he felt “pretty shell-shocked” by the collision
“The word that keeps coming to my head is just brutal really.
“Just going from, I don’t know how fast we were going, maybe 40, 50, 60mph (64-96km/h) , to nothing in the blink of an eye,
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