Sheriff Brian Lockhart, in a report into the crash on the Newton to Kirkhill line, near Glasgow, said rail management was also to blame for the accident which could have been avoided if ScotRail had kept a double track on the line.
He said the line was switched to a single line - on which trains alternately ran in opposite directions - weeks before the accident.
The sheriff said the driver, Reg McEwan, 61, caused the crash on 21 July 1991, by passing a red signal and taking his train on to the single-line stretch where it met a train coming in the opposite direction.
The crash was the highest speed collision in British Rail's history - the two suburban electric trains ploughed into each other at a combined speed of up to 70mph. Both drivers and two passengers died and 22 other people were injured as the trains telescoped into each other.
The 63-page report followed a fatal accident inquiry at Glasgow Sheriff's Court. A signalman, Edward Dillon, who was making tea at the time of the crash, was absolved of any blame.