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Unprecedented failure led to disaster

News - Potters Bar Rail Crash

DEATH AND disaster has once again visited Britain's railways, all because two pairs of 50mm diameter nuts were missing from two 'stretcher bars' essential to the workings of a points set.

The ill-fated points 2182A at Potters Bar were installed in 1 994. Head of the Health & Safety Executive's Railway Inspectorate (HMRI) Dr Allan Sefton told NCE that the points were of a standard design, and had no unusual features. Around 2,000 such points exist across the UK network.

The points consisted of two fixed 'stock rails' and two moveable 'switch rails', tapered to fit against the web of the stock rails when switched by electric motor (see diagram).

The points set had a 'front' stretcher bar - also known as a locking bar - and a 'first' and 'second' stretcher bar. The front stretcher bar - a flat steel section - holds the ends of the switch blades in position. It also performs an essential function as part of a signalling circuit, telling signal controllers if the points are in the correct position. The bar is adjusted to tolerances of less than 1mm.

The first and second stretcher bars are square steel sections with rounded threaded ends. They keep the switch rails apart and are bolted to brackets fixed to rails.

The end of the first and second stretcher bars are secured by two nuts and two locking nuts. HMRI investigators said that the outer two bolts on the left hand first stretcher bar and the two outermost on the right hand side of the second stretcher bar were loose. They were found underneath the bars after the tragedy (see diagram).

With the nuts off the bars, the switch rails were free to move independently.

An expert involved in the investigation said that horizontal forces exerted by the high speed train moved the rails into the wrong position as the Class 365 Kings Lynn train's third carriage passed over it. The gauge of the points narrowed, forcing the wheels off the track.

He told NCE that the failure was unprecedented. He said removal of the missing nuts was highly unusual, as only minor adjustments of the bolts would be needed to keep the correct distance between the switch rails.

He said the vibration working the bolts loose was 'highly unlikely' as it would require the bolts to travel a considerable length of threaded bolt. Removal of the nuts would only be necessary if they or the bars or insulating bush needed to be replaced.

Former British Rail headquarters permanent way engineer Geoffrey Cole told NCE that these bushes performed a key role: 'If the insulation resistance is lowered, then you get spurious indications that the rail is occupied when it is not, the points are locked and the signals are red.' However, he said it was 'mind boggling' if bolts were removed and not replaced.

A spokesman for Jarvis said that one of its three-man signalling maintenance teams found two bolts on a stretcher bar at points 2182A missing on 1st May, and replaced them. A normal weekly visual inspection on 9th May noted no problems.

The points underwent a joint Jarvis/Railtrack detailed 'Galaxy' test in March, and a regular detailed quarterly check also in March.

'We have done everything we were required to do under the procedures, ' he said.

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