Background
Dierdorf is a town of 5741 people (as of December 2019) in the western German federal state of Rhineland-Palatine, located 46km/29mi southeast of Bonn and 20km/12mi north of Koblenz (both measurements in linear distance).
The town is passed on its southern edge by the Cologne-Frankfurt high speed rail line, a 180km/110mi double-tracked main line opened in 2002. Engineered to allow top speeds of as much as 300kph/190mph the new railway line drastically cut down travel time between the two cities. The line, which is an important corridor of national and international passenger services, runs closely bundled with the Autobahn A3 for most of its length and has an unusually high maximum incline of 4%, requiring trains with a relatively high power to weight ratio. Due to the unusual combination of high speeds and relatively high inclines the line is also used for testing of new trains. The line was the first extensive use of a “ballastless track”, meaning the tracks were laid on concrete rather than conventional gravel.
The train involved
On the day of the accident ICE 511 from Cologne to Munich consisted of a double-traction of third generation ICE high speed trains (listed as DB series 403) with unit Tz 351 leading and Tz 326 (the affected train) following. The ICE 3 is an eight-car electric multiple unit introduced into service with the DB (German national railway) in 2000. Weighting 459 metric tons at 200.84m/659ft in overall length the ICE 3 (series 403) can reach 330kph/205mph thanks to a power output of 8000kw/10728hp. Doing away with the predecessors’ dedicated motor cars the ICE 3’s drivetrain is spread through the train underneath the interior floor, with each train consisting of two “traction units” of four cars with 8 motors each. The traction units consist of an end car, a transformer car, a power converter car and an uninvolved middle car. Only the end cars and converter cars are powered, giving the 8-car train 4 powered cars in two independent systems. Tz 326 entered service in February 2001, being christened “Neunkirchen”, and could carry a total of 454 passengers in a two-class configuration. Usually an ICE 3 carries at least 5 crew members, but as it was being pulled by another unit on the day of the fire Tz 326 didn’t have a driver on board.
The accident
On the 12th of October 2018 ICE 511 is travelling eastbound through Germany on its way from Cologne to Munich, carrying 506 passengers and 4 crew members. At 6:21am the train is approaching Dierdorf at 270kph/168mph as the onboard control system registers an electric overload at the pantograph. The train’s protection system, assuming a defect in the overhead catenary, shuts off the main power switch. Unbeknownst to anyone on the train the cause is actually a fire that started at the rear transformer of Tz 326. Over the following minute the driver turns the main switch back on several times (as instructed), being met with the same automatic shutoff each time despite the leading train reporting no defects. By 6:23:30am the speed has reduced to 208kph/129mph. At this point an off-duty officer of the German federal police in the second to last car of Tz 326 sees smoke emit between his and the last car and triggers an emergency stop. At the same moment the burning transformer’s Buchholz-relay reports an error, causing the train’s control system to keep the rear train’s main switch from being turned on again. Only now does the driver learn of the fire in the rear train. By 6:24:09 the train comes to a stop outside Dierdorf, moments later flames start licking up the sides of the second to last car.
The train crew finds the loudspeaker-system on Tz 326 inoperable, as such they have to go from car to car and instruct the passengers to evacuate the train to the right, away from the opposite track. By the time the train crew makes their way through the increasingly smoke-filled train to the second to last car passengers have already evacuated on their own as flames reached the interior of the train and eat their way through the car in intense heat. The train driver, being notified that the train is empty, instructs dispatch to alert the local fire department before abandoning the train also. By the time the last passengers leave the train the fire department is already arriving, by now half the affected train car is ablaze. Thanks to quick reaction by passengers and crew no one dies in the fire, only five people are treated for minor injuries at the local community center. Once the DB confirms that the overhead catenary is turned off firefighters start to battle the aggressive fire, which soon expands to the rear car of the train. The thick black smoke can be seen from a long distance away.
Aftermath
The fire department eventually gains the upper hand and manages to extinguish the flames, by the time the investigators are cleared to approach the wreckage little is left of the train car the fire originated in. The interior, roof, windows and most of the walls are gone, remains of molten cables and seats spread across both tracks. The rear train car couldn’t be saved from the fire either, while smoke and firefighting-foam cause so much damage to the rest of Tz 326 that it will later be written off. Investigators are now faced with the difficult task of explaining how and where the fire started while most of the point of origin is gone. Based off the interior materials, witness statements and traces on what’s left of the rear train they soon rule out arson by a passenger, turning their attention to the train’s electrical system which is located along the floor of the train car between the wheel sets. Third generation ICE 3 had seen their transformer-car catch fire twice before, in both cases the cause had been traced to short circuits at the main transformer. As such the investigators, once the train is moved from the site to a maintenance yard, start pulling apart what’s left of the burned car’s underpinnings. Due to the degree of destruction the investigators blocked an identical train from being released back into service, using its electrical system for comparisons.
The ICE 3’s main transformer sits in a bolted shut oil filled steel box unluckily called the “Trafokessel” (“Transformer boiler”), the unit removed from the burned train car showed large dents in the walls pointing to a rapid increase in oil pressure inside the box. The report notes that there were traces of high heat damage, but no evidence of active fire inside the box once it was opened. The investigators found the oil nearly drained from the box, and electrical burns on an oil line and an aluminum shield inside the box pointed to a short circuit inside the transformer.
Parts of the high-power side of the transformer showed severe damage, with parts of the coil deformed and loosened. In some places the isolation had been destroyed.
The transformers are held in place in their boxes by six tension rods (2 rows of 3) keeping them immovable between two girders. As they pulled the transformers out of the box investigators found that the center tension rod in the lower row had snapped in two right next to where it was bolted into the girders, reducing tension on the transformer and allowing a few centimetres of motion.
Inspecting the snapped rod showed that it had been refurbished, which was not allowed, as it displayed cut rather than pressed threads. Who had done that at which point for what reason couldn’t be found out.
Next the investigators turned to the overload-ground, a wire which leaves the trafo-box through a watertight cast resin sleeve to a ground-point outside the box. The wire was found hanging out of a shattered sleeve in a badly burned state, with traces outside the sleeve’s location indicating that the fire had originated close to this point.
At this point investigators were able to form a theory on the development of the fire. The tension rod had broken due to fatigue (as the cut thread had reduced its diameter too much), allowing part of the transformer to angle down and touch an oil line and an aluminum shield located below it inside the box. The ensuing short circuit caused a near-instant gas formation which increased the pressure inside the box (enough to dent the ouside walls of the steel box). The increase in pressure ripped the resin sleeve out of its place, allowing oil to spray out of the trafo box. The entire process from the short circuit to the oil spraying out took 10 miliseconds, too fast for the Buchholz-relay to cut the oil pumps and shut off the train due to excessive oil pressure. The oil then sprayed onto hot components outside the trafo box and was ignited (with the sprayed mist character making it much more flammable than usual). The fire destroyed the Buchholz-relay, keeping it from working or being able to be examined by investigators. It took almost a minute for the train’s onboard diagnosis system to register an unusual feedback from the (long gone) relay, triggering the same stage 2 alarm it was meant to send in case of sudden overpressure, and permanently shutting off Tz 326. By that point the emergency stop had already been initiated.
With the cause of the fire clear the DB announced that they were going to examine the transformers and surrounding systems on all ICE 3 and to reduce the inspection interval for the transformers. Investigators had found metal dust in the little oil that remained in the affected train car, but while it was inconclusive at the time they later suspected that it was an early sign of fatigue cracks working their way through the tension rod. Finding the same dust on 4 other ICE 3 trains made the DB pull them from service. The unscheduled checks meant the DB was lacking up to six trains per day, and trains that were yet to be checked were banned from travelling through long tunnels. All trains travelling on the line have NBÜ-systems (Notbremsüberbrückung/Emergency stop override) allowing the train driver to overrule emergency stop orders in a tunnel, but the DB still didn’t want to risk a fire in such a confined space. ICE 3 don’t have any fire detection system, relying on monitoring places that could see a fire start by means of temperature and pressure. The DB also declared that the loudspeaker-system aboard trains now had a higher priority, it had been found to be defective but not been seen as important to repair immediately at least once prior to the accident. Instead a megaphone had been placed aboard the train for the staff to use. On the day of the accident the train wasn’t at capacity and the passengers were very calm and organized in leaving the stricken train even before the staff could assist them, one does not want to imagine how things could have gone on a full train and/or had a panic started among the passengers. Lastly, it was criticized that the driver could turn the main switch back on as quickly as 3 seconds after a yet-unknown defect triggered it. This allowed sustained power to be sent to the defective transformer, keeping the short circuit going.
After the accident Tz 326 was stripped for parts and later scrapped, damage from the fire and the firefighting effort along with smoke contamination put the unit beyond economical repair. The leading train soon returned to service once the investigation cleared it, it had remained completely undamaged in the fire. The affected section of the high speed line remained completely closed for several days as the DB repaired damage to the track, signaling system and overhead catenary. In December 2018 the DB introduced the fourth generation on the line, due to those trains’ lower top speed they are only used on selected connections with fewer stops along the way.
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