The report said work was being staged, couple of weekends apart.  The new signalling was installed correctly, but a feed wire to the removed signalling was just undone and left loose (it was not removed because circuit was powering a part that had not been changed).  Someone knocked this bare wire and it came into contact with the signals in question giving false display. 

The accident came down to lazy approach of not insulating an old wire that was retained temporarily and was going to be stripped out few weeks later.  Ironically had the work all been done in one big changeover as had been done in 1936 when the signalling was changed to colour lights, the old circuit would have been dead and no wrong signals displayed.

The trains did not have anti climb units (serrated panels that engage) on ends of underframes, were old mk1 design with separate underframes, no bright taillights which reflect off the rails (just red bars in headcode box), and had no emergency radio to signallers unlike today.   So lot has changed.

 
I remember being so shocked at this accident.  Watching the pictures on tv news left me feeling sick and numb.  How on earth could this have happened?

I think it turned out that recent upgrade work in a nearby S&T cabinet had not been done properly and hadn't been checked and tested.  This led to big changes in the process and culture of new works being done on the railway

 
Thank you for bringing us a separate thread on that dreadful morning ... I have amended "on this day" so that it now points direct to this thread.  I remember the shock at the time - not that I had any public transport involvement in my life at the time; it took over the general news.   Thank goodness, at least, that lessons were learned and we now have a safer railway resulting from some of the changes it brought about in, in memory of those who lost their lives.

 
On 12 December 1988 the 07:18 from Basingstoke to London Waterloo, a crowded 12-car train made up of four-car 4VEP electric multiple units was approaching Clapham Junction when the driver saw the signal ahead of him change from green to red. Unable to stop at the signal, he stopped his train at the next signal and then reported to the signal box by means of a line-side telephone. He was told there was nothing wrong with the signal.
At 08:10,the following train, the 06:30 from Bournemouth, made up of 4REP unit and 2 4TC units, collided with the Basingstoke train.
A third train, carrying no passengers and comprising 2 4VEP units, was passing on the adjacent line in the other direction and collided with the wreckage immediately after the initial impact.
The driver of a fourth train, coasting with no traction current, saw the other trains and managed to come to a stop behind the other two and the signal that should have protected them, which was showing a yellow aspect instead of a red aspect.

I can remember this very vividly in my early railway career, the "Hidden" enquiry lead by Anthony Hidden, QC, resulted in significant changes in how railway infrastructure, in particular signalling, operation, renewals and maintenance were managed and organised.  It brought in the max 12 working hours and limiting 13 days working in 14, it was not uncommon at the time for railway staff to work 16 hour days without any days off; to many railway(wo)men it was to only way at the time to earn a decent wage.

There were many other changes brought in by Hidden which to this day impact on how the railways are operated, renewed and maintained

We should remember the 35 who died, the 69 seriously injured and the other 415 sustained minor injuries; and the emergency services and railway staff that responded to the incident