Although the official report into this accident has not yet been published the following explanation is being given on a number of websites:-
The signalman accepted two trains, one in each direction and closed the crossing barriers (which were not interlocked with the signalling/track circuiting) before clearing his signals.
Whilst one train was passing he answered a phone call relating to one of the other crossings under his control. As a result of this distraction he forgot about the second train (which hadn't yet passed) and re-opened the barriers.
He quickly realised his error and placed his signals at danger but despite an emergency brake application the approaching train collided with two cars.
It is also being reported that an offer to install barrier control interlocking during a recent equipment overhaul project at this signalbox was rejected by Network Rail managers.!!

Iraq medical students including prof were found dead when train fell down the canyon. The most sad story ever heard.