Just before 07:00 hrs on Friday 16 September 2016, a London-bound passenger train operated by London Midland struck a landslip at the entrance to Watford slow lines tunnel. The leading coach of the 8-car train derailed to the right. The train came to a halt in the tunnel about 28 seconds later with the leading coach partly obstructing the opposite track. About nine seconds later, the derailed train was struck by a passenger train travelling in the opposite direction. The driver of the second train had already received a radio warning and had applied the brake, reducing the speed of impact. Both trains were damaged, but there were no serious injuries to passengers or crew. However, had the first train been derailed only a short distance further to the right the consequences would have been much more severe.
The landslip occurred during a period of exceptionally wet weather. Water from adjacent land flowed into the cutting close to the tunnel portal and caused soil and rock to wash onto the track. The site had not been identified by Network Rail as being at risk of a flooding-induced landslip. Such a landslip had occurred at the same location in 1940, also causing a derailment. Drawings from the 1940s relating to a structure subsequently constructed to repair the slope were held in a Network Rail archive, but were not available to either Network Rail’s asset management team or the designers of a slope protection project which was ongoing at this location at the time of the accident. As a consequence, this project made no provision for drainage.
Both trains were crewed by a driver and a guard. The drivers each contacted the signaller to inform him of the accident and request the evacuation of passengers. The guards checked on their passengers to confirm that there were no casualties, and made regular announcements to keep passengers informed.
The RAIB has made six recommendations. Four recommendations are addressed to Network Rail relating to the improvement of drainage, improvement in the identification of locations vulnerable to washout, access by the emergency services, and to expedite a project intended to identify all drainage assets. One recommendation is made to the Rail Delivery Group, in conjunction with RSSB, to promote a review of the circumstances when bogie or infrastructure design could provide derailment mitigation. One recommendation is made to Siemens, the manufacturer and maintainer of the trains, to address issues relating to the securing and location of emergency equipment which came loose in the driving cabs of both trains when they collided.
The RAIB has also identified three learning points relating to issues identified during the investigation.