The Rail Accident Investigation Branch have released their report into the derailment at East Croydon back on 17th February 2012 with the cause of the incident being put down to issues with the track circuit which meant that the points moved as the tram was crossing them to enter the tramstop.
The incident occurred at approximately 0623 on Friday 17th February 2012 when Bombardier built CR4000 2538 was approaching East Croydon tramstop from the east just behind another tram. Following the clearance of the other tram from platform 3 (the normal westbound platform at East Croydon) 2538 was routed towards the same platform as per normal practice. Unfortunately as 2538 was passing over the points which allow trams to either access platform 2 (the centre platform) or platform 3 they changed leading to the centre and rear bogies being routed towards platform 2. The centre bogie subsequently derailed and the tram was bought to a halt. There were approximately 100 passengers on board the tram at the time with no reported injuries as it was only travelling at low speed.
Following the RAIB’s detailed investigation they ruled that the main cause of the accident was that “a track circuit had failed to respond to an approaching tram and lock the points to prevent movement”. As the tram in front had stopped short of the signalling loop at East Croydon tramstop it was not until it departed the platform that the signalling equipment realised there was already a tram in platform 3 which then led to the points being changed to lead to platform 2. Unfortunately by the time this instruction was made 2538 had started to cross the points which were in the process of moving as the tram approached. The track circuit should have noticed the presence of 2538 which would have prevented the movement of the points but this did not happen which led to the derailment. It was discovered that the track circuit had been incorrectly adjusted and that the rail head “may” have been contaminated with silt (this could have been a reason why the track circuit failed to identify the presence of 2538). The RAIB also believe that the system integration was inadequate as at the time Tramlink standards did not adequately identify how the correct relationship between vehicles and infrastructure should be tested and maintained. Maintenance of Tramlink had not been maintained as an integrated system and the standards to which the point controller and wheel tyre to wheel tyre resistance were being maintained were not sufficient to maintain the correct relationship between the two elements of the system.
Since this incident occurred London Tramlink purchased dedicated maintenance equipment to measure the performance of track circuits. The nine track circuits across the system controlling facing points have been adjusted to respond when a 0.3 ohm test resistance is applied to the rails which should hopefully avoid any track circuits now etecting a tram as happened in this derailment. Track circuits are now also subject to routine testing. In addition London Tramlink have now taken direct control of TOL’s contract with Bombardier and a Chief Engineer has been appointed with responsibility for maintenance standards (although this change was in the process of happening anyway at the time of the incident).
As a result of the investigation the RAIB made three recommendations to help ensure a similar incident does not happen again:
- London Tramlink should review the operational and signalling arrangements at East Croydon to consider whether too much reliance is placed on the track circuits operating correctly.
- London Tramlink should identify any areas of paved track where silt collects and an improved inspection and cleaning regime should be introduced where this may affect the safe operation of the tramway.
- A fundamental review of track circuit settings and wheel tyre to wheel tyre resistances should be undertaken and following this a system of maintenance to ensure these work correctly should be introduced.
* You can read the full RAIB report (including photos, diagrams and a lot more – technical – detail) on their website at http://www.raib.gov.uk/cms_resources.cfm?file=/130328_R042013_East_Croydon.pdf