Lessons from the wreckage

2019-03-07 09:01:17

By Mick Hamer IN THE wake of last week’s horrific London train crash, safety experts are wrestling with two key questions: what steps can be taken to prevent trains passing through red signals? And why did one of the coaches involved erupt into a fireball? Safety specialists have complained for years that Britain has lagged behind other developed nations in introducing automated systems to prevent trains passing through signals set to red. So in that respect, last week’s tragedy did not come as a big surprise. But the ferocity of the fire amazed experts. “It’s taken everyone by surprise,” says Rod Smith, who heads the Advanced Railway Research Centre at the University of Sheffield. “We’d assumed fires went out with wooden carriages and oil lamps.” Establishing why the fire was so devastating will be a key task for the public inquiry into the disaster, headed by the Scottish judge Lord Cullen. But experts contacted by New Scientist—including an engineer who was a passenger on one of the trains involved—suggest that the friction of a steel high-speed locomotive crunching through an aluminium-shelled local train would have generated enough heat and sparks to ignite the diesel fuel spilt in the collision. More than 30 people are now thought to have died in the carnage at Ladbroke Grove Junction, west of London’s Paddington station. Some suffered severe burns after fire engulfed the second coach of the high-speed train, operated by Great Western, which was on its way into Paddington. The local train, operated by Thames Trains, failed to stop at a red signal as it was heading out of London. As a result it crossed into the path of the high-speed train, meeting it almost head-on (see Diagram). Both trains were diesel powered. The London-bound express, travelling at about 110 kilometres per hour, was powered by a locomotive with a single fuel tank underneath. All three carriages of the Thames train, travelling at about 70 kilometres an hour, had fuel tanks beneath. Several of these tanks are thought to have ruptured in the crash. Eyewitness accounts provide clues to why the spilt fuel ignited so rapidly. A consulting engineer who was on the Thames train recalls hearing two impacts. The first seemed like a glancing blow. Nevertheless, after this impact, the express locomotive peeled the roof off the first carriage of the Thames train and ripped it apart. The engineer, who does not wish to be named, then heard a loud bang when the second carriages of the two trains crashed into each other and shuddered to a halt. Several small fires immediately broke out on the Thames train, as newspapers ignited, although these didn’t spread. “It was like a film set,” says the engineer. Based on this account, Andrew Gardiner of Arup Fire in London, a fire safety consultancy, says that the friction generated by the high-speed locomotive shearing through the first carriage of the Thames train could easily have warmed diesel fuel to its flash point of around 54 °C. The presence of sparks and the possibility that spilt fuel came into contact with a hot engine may have contributed to the blaze. But had the collision caused less friction, there might have been no inferno. So could anything be done to prevent such conflagrations? While it has proved possible to engineer racing cars so that they don’t catch fire in a crash, Gardiner doubts that similar technology could be employed on the railways. The energy involved in a collision between trains weighing hundreds of tonnes is simply too great, he says. Of course, there would have been no crash and no fire had the Thames train not jumped a red signal. Many experts agree that the long-term solution is to introduce an automated system to bring errant trains to a halt—as a number of countries have already done. But this will take years to install. In the meantime, a report delivered to Railtrack, the company responsible for the track and signalling on Britain’s railways, suggests that safety could be improved by making sure that problematic signals are positioned where they are clearly visible and sunlight can’t confuse train drivers. The report, by Alistair Gale of the Applied Vision Research Unit at the University of Derby, was completed in 1994. Signal SN109, implicated in last week’s disaster, was a known black spot. Trains passed it at red eight times between 1990 and 1998. SN109 is partly obscured by an overhead gantry. Difficult-to-see signals often have “repeaters”—a back-up signal showing the same colour so that drivers are left in no doubt—but SN109 did not. SN109 is by no means the most dangerous signal on Britain’s railways, however. Ten signals have a worse record, and one at Swinton in South Yorkshire was passed at red 16 times in the same period. Gale also noted that bright sunshine can be a problem. Railway workers say that sun can make signals glint and appear to show the wrong colour. Whether this was what confused the driver of the Thames train we may never know. But the crash did occur on a bright morning when the sun was low and shining into the signal. For more on the technology that could prevent such crashes,