Background
Denguin is a municipality of 1766 people (as of 2019) in the far south of France, located 80km/50mi east of Bayonne and 60km/37mi north of the Spanish border near Jaca (Spain). Both measurements are in linear distance.
Denguin lies on the Toulouse-Bayonne Railway, a 319km/198mi double-track electrified main line which is one of France’s most important railway lines. Opening in sections between 1861 and 1867 the line was an early candidate for electrification, gaining the wires after WW1 to help against the high coal-consumption of the trains on the mountainous line. Originally carrying passenger and freight service the line gained high speed trains in the 1990s with the introduction of the TGV-system to the area. Today the line sees 30–40 trains per day, mostly regional passenger services, at speeds of up to 160kph/99mph.
The trains involved
TGV 8585 was a high speed passenger service from Tarbes to Paris, provided on the day by unit 318. Unit 318 was an “Atlantique”-Series, a 12-car electric multiple unit introduced in 1989 and refurbished between 2005 and 2009. The Atlantique is the second generation of the TGV high speed trains and likely the most famous one with their sleek design and silver paint job. Each Atlantique measures 237.5m/779ft in length at a weight of 444 metric tons empty. The trains consist of 10 unpowered passenger cars riding on shared so-called Jacobs Bogies with a dedicated four-axle motor car on each end. The Atlantique’s motor cars have a combined power output of up to 10400kW/13947kW and run at up to 320kph/199mph with a modified and shortened train reaching 515kph/320mph for a world record.
Travelling behind the TGV in the same direction was TER 867285, a regional train from Tarbes to Dax. On the day of the accident the connection was provided by TER Z7364 (TER is the French national Railway’s regional express brand), an SNCF (French national railway) series Z7300. Introduced in 1980 the Z7300 is a two-car electric multiple unit that seats 151 passengers in a two-class configuration. Each two-car unit measures 50.2m/164ft in length ata weight of 104 metric tons. They can reach up to 160kph/99mph and are part of a whole family of “Z2” multiple units made in the early 1980s for the SNCF. At the time of the accident the two trains carried a total of 228 passengers, about half the TGV’s capacity alone (other sources mention 238 passengers).
The accident
On the 17th of July 2014 at approximately 5:15pm TER Z7364 is travelling westbound through the countryside, approaching the town of Denguin (which has no station on the line). The train carries just 50 passengers and a driver as it rumbles along the track at 130kph/81mph. Just outside Denguin the regional train gets a stop-order, coming to a halt at Signal S23 as the block ahead is supposedly still occupied. After a short wait the signal turns green and the driver accelerates again. Unbeknownst to him the TGV ahead of it was travelling at just 30kph/19mph, driving “on sight” as Signal S23 had been out of order when the driver had passed it. Within minutes the regional train caught up to the TGV, reaching 128kph/79.5mph by the time the driver spotted the slow high speed train in his path.
He immediately triggered an emergency stop, bringing the train down to 95kph/59mph in the final moments before impact. At 5:30pm the regional train struck the rear motor car of the high speed train at an excess speed of 65kph/40mph in what must’ve come as an absolute surprise to the until-then unaware occupants of the latter. The impact severely damaged the driver’s cab of the regional train, caving in the leading wall and pushing the control desk back while the TGV’s rear nose cone was largely obliterated by compression against the unoccupied rear cab, shortening the motor car by a few feet and leaving it with a near-vertical end. As the two trains came to a stop at a level crossing north of Denguin everyone aboard was alive but 40 people were injured, 5 of which (including the regional train’s driver) severely.
Aftermath
Both drivers independently notified their superiors about the accident just moments after the trains had come to a stop, who then alerted emergency services about what had happened. A few minutes after the accident responders were on site and combed through the passengers, most of which had evacuated the trains on their own. Only a handful of survivors needed help to leave the stricken trains, in the end 5 people were taken to hospital due to the nature of their injuries, 3 of which by helicopter. Another 33 people went to the hospital on their own/were taken to hospitals at a lower priority. Before the day was over the trains were cleared of occupants and all passengers and crew were taken care of, leaving the site to the investigators.
Originally investigators suspected human error, theorizing that the regional train’s driver departed early after stopping at the red signal S23 without permission. The trains had no pre-existing defect and the system correctly detected the TGV. Then why did the signal turn green, allowing the regional train to proceed at regular speed after just a short pause, instead of holding it until the TGV was a safe distance down the line? Every modern rail line in the world is split into “block zones”, sections of track separated by signals. When a train enters a new block-zone the system detects it and turns the signal behind the train red until the train has left the section for the next one. It’s an automatic process with no-one manually flipping a switch to turn a signal red or green.
Had the regional train really run a red signal the train control system should have detected the unauthorized entry into the block-zone past S23 and triggered an automatic stop, keeping the trains from getting too close to one-another. While this system can be overridden for shunting or during malfunctions of the signaling systems (in which case the entry has to be authorized by the local signal box crew) none of that had happened ahead of the collision at Denguin. The status of the system as it presented itself to the investigators was that of perfectly regular operation of both the signals, the train control system and the regional train’s brakes. At that point the regional train’s driver was relieved of guilt as he physically couldn’t have run the signal, caught up to the TGV and crashed into it on purpose, which was backed up by him triggering an emergency stop about as soon as the TGV came into view. Furthermore, he couldn’t have known that the TGV was there, travelling at the unusually low speed.
Investigators soon turned their attention to the systems located at S23, consisting of the signal itself, an axle-counter to ensure a block was empty, and a simple metal shed standing in the grass next to the rail line. On French rail lines used at 160kph/99mph or less axle counters count the axles of a train entering a block zone and then again at the end of the zone, ensuring the block zone is empty if the number entering and exiting the zone is equal. In regular operation the signal to enter the block zone won’t turn green unless the numbers match.
The metal shed holding the electrical systems for the signal and axle counter is a rather flimsy construction found thousands of times all over France, its main purpose being protection from the elements, not necessarily being a sealed environment. There was a history of workers finding plants growing inside the sheds, panels fitting so poorly that you could see a clear slit between them or insects like wasps or ants choosing the environmental protection to built nests. Inspecting the shed at S23 investigators found poorly fitting panels and plants inside the shed, looking further they stumbled upon what would be a breakthrough.
They found a rodent’s nest made up from grass, twigs and ripped up paper belonging to a 2008 technical diagram, along with urine and feces pointing to rats or mice having made a home in the shed. The concrete slab the shed was standing on had broken and partially sunken, creating a gap of over 20mm at the bottom of the shed when 15mm would be enough for a rodent to slip through. Lastly, they found several dozen wires with bite-marks matching those of rats, to the point that some cables’ insulation was missing in chunks. The relay controlling S23 was also faulty, having been installed tilted to the left and improperly locked. Dirt on the relay support below it also showed that the systems in the shed had not yet seen the annual inspection, but at the time SNCF guidelines allowed a 90-day delay on those.
Following various wires and examining the damage to them investigators soon worked out that the often shrugged-off lackluster condition of the sheds had led to the collision. Apparently rats had made a home in the shed and in the process nibbled the insulation off various cables, including ones that ended up making an unintended connection, overriding the axle-counters and turning S23 green just as the regional train was waiting at it. In fact, it can’t be said that there had no so-called wrong side signal failure before this with just the configuration of present trains and speeds avoiding a collision. Had the two trains on the day of the accident run at their regular speeds they would have stayed apart despite the wrong signal being shown to the regional train, with the next block section’s signal likely holding the regional train properly until the TGV was safely ahead again. The report doesn’t say if the malfunction that caused the TGV to travel at the reduced speed was caused by the rodent damage also.
The day after the accident the SNCF launched an effort to examine and, if needed, fix the condition of 37000 out of 41000 signal tech sheds similar to or identical with the one at Denguin. The results were presented as follows:
- 36% of the sheds showed signs of rodents at least passing through them
- 5% of the sheds had rodent damage to their wiring
- 15% of sheds showed deteriorated wires without being below the official standard
- 1% of sheds had wiring in a condition that required immediate action
This meant that the damages found at the site of the accident were no exception and that the French railway had been sitting on several more or less urgently ticking time-bombs.
After the results of the examination the SNCF announced that around 3000 sheds would be refurbished by the 10th of August that year and that the maximum allowed delay of the annual inspections would be shortened. An introduction of a more secure shed-design was not pursued.
The accident had no legal consequences for anyone involved, with SNCF paying damages to the survivors depending on the severity of their injuries and the consequences of them. Some of the survivors refused the offered money, choosing to take SNCF to court instead. One of them, Mister Warryn, said in an interview a year after the accident that the offered (undisclosed) sum was too low and didn’t consider mental consequences, and that it also was “a matter of principle” to not let the SNCF get off the hook too easily for something caused by professional negligence. Unfortunately, the outcome of the individual trials is unknown. In the same piece, published by a French radio station, it is also said that many train drivers are still affected by the accident, getting worried about what might happen any time they hear about “a malfunction” or “an incident” as it reminds them of the collision at Denguin and a derailment at Brétigny the year before which killed 7 passengers after a train derailed on a faulty piece of track. The union criticized the deteriorating maintenance of the infrastructure, which would need inspections every 2–6 months rather than upwards of a year.
Today the SNCF Series Z7300 is nearing the end of its service life, being gradually replaced by more modern multiple units like the Series B82500 introduced in 2004.
The severely damaged rear motor car of TGV 318 was withdrawn from service and scrapped after the accident, with the train returning to service with a different motor car at the rear. The Atlantique-model is still in regular service today along side the newer bilevel “Duplex”-units and no fixed retirement-date is in sight. However, in 2018 the SNCF ordered 100 units of the “Avelia Horizon”, a successor to the TGV-platform, with deliveries set to begin by 2023. Oddly the new trains will stick to the layout of unpowered passenger cars and two dedicated motor cars, something most high speed trains have since departed from.
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