Background
Fritzens is a town of 2189 people (as of 2024) in western Austria, located 15km/9.3mi east of Innsbruck and 41km/25.5mi west-southwest of Wörgl. The town has its own train station, but is also located above the Terfen Tunnel, a 16km/10mi double-tracked railway tunnel opened in 2012.
The Terfen Tunnel (“Terfener Tunnel”, named after the town of Terfens above its middle) is part of the new Lower Inntal Railway, itself part of the northern Brenner approach, a rail corridor named after the 1270m/4495ft Brenner Pass that it sees trains cross. The old Inntalbahn, which also runs through Fritzens, was no longer up to the demands of rail traffic regarding speed and capacity, and it was thus decided to construct a new main line bypassing the old Lower Inntal Railway, which was built largely underground in order to both shield the line from the environment but also protect the area from the noise of the rail traffic. Once complete by the mid-2030s the new rail corridor will cut Munich-Innsbruck travel times by an hour and Munich-Verona travel times (where it compliments further new construction projects) by 3 hours.
The Terfen Tunnel consists of a single tube holding two electrified rail lines built on a “fixed track”, meaning the rails are bolted to a concrete path rather than individual concrete beams sitting on gravel. Electrification is provided by an overhead catenary at 15kv, the Austrian standard. The tunnel is used by both freight and passenger services at speeds of up to 220kph/137mph, significantly faster than the old route on the surface. Several “Rettungsstollen” (“Rescue Adits”) offer emergency access to the tunnel from the surface via staircases while also providing ventilation.
The Vehicles Involved
Nightjet 40420/420 is an international express passenger service from Innsbruck (Austria) to Hamburg (Germany) and Amsterdam (Netherlands), with the train being split at Nürnberg (Germany). Throughout 2023 the train usually consisted of nine four-axle express passenger cars from the ÖBB (Austrian national railway), split between standard intercity- and sleeper-cars, along with three ÖBB type DDm bilevel car carriers allowing passengers to bring their car along. The locomotive is on the car carrier-end of the train until Munich (Germany), where it has to switch to the opposite end of the train.
The DDm-type car carriers were introduced into service in 1971 and can carry five passenger cars on each level. They have an empty weight of 27.8 metric tons. While they offer a popular option for Nightjet-Customers their use also limits the trains’ top speed to a mere 160kph/99mph, while the other train cars can reach 200kph/124mph. Newer second-generation Nightjet trains are faster still, doing away with the car carriers for that advantage.
One of the cars loaded onto the car carriers on the day of the accident was a second generation Mercedes-Benz Vito, a van with a camping-conversion. The Vito Mk2, internally labeled W639, measures 4.75m/15.6ft in length and 1.87–1.94m/6.1–6.4ft in height depending on configuration. The camper conversion involved in the accident, like many of its type, had been fitted with a pop-up roof to increase interior height and/or offer additional space to sleep without having the drawbacks of a permanently raised roof (which, for example, wouldn’t fit on the car carrier). Pop-up roofs are new roof-pieces, attached after the stock roof is cut away, usually being hinged at the rear with fabric on the sides and front unfolding into walls when the roof is up.
The Accident
Nightjet 40420/420 has barely left Innsbruck station on the 7th of June 2023 by the time it dives into the western portal of the Terfen Tunnel, travelling at 160kph/99mph with 150 passengers onboard. The train is just a few hundred meters past the portal when the train suddenly registers a loss of power from the overhead catenary, triggering an emergency stop which brings the train to a halt below the town of Fritzens at 8:50pm. The train driver reports the emergency stop and, before the dispatch center can do much to handle the stranded train, adds that a fire has been spotted on the train. One of the car carriers at the front of the train is burning, filling the tunnel with significant amounts of smoke. Firefighters from four departments rush to the tunnel, all of them have received special training for incidents like this. They reach the tunnel at 9:00pm, setting up at three emergency access points where they have to wait for the overhead catenary to be shut off and grounded so they don’t risk a fatal electric shock. A first group finally enters the tunnel near the near the front of the train, finding the tube completely filled with smoke leaving essentially zero visibility.
The passengers, meanwhile, are stuck aboard the train, with the train cars’ construction keeping the toxic smoke outside but also meaning they can’t leave. They later recall hearing repeated detonations echo through the tunnel, what might make one think of explosions are actually the tires of cars popping in the heat. The first group of firefighters starts fighting the blaze centered around two burning cars on one of the car carriers while others set up ventilation, aiming to push as much of the smoke out of the tunnel as possible by means of large movable fans which support the tunnel’s built-in ventilation. Soon over 700 responders are involved in the rescue effort. Tunnel fires are tackled in opposite order to a house fire, with extinguishing the fire taking priority over evacuating civilians. In a house fire the fire department wants to get people out of harm’s way, but smoke and especially heat are a much bigger issue in a tunnel fire due to the enclosed space. Smoke aside, the air in a tunnel can reach over 100°C near a fire. Firefighters are somewhat protected by their clothing, but the civilians they’re evacuating aren’t. Thus, if possible, the fire is to be extinguished first, limiting the escalation of the temperature but also stopping smoke-production.
Firefighters could follow that procedure on that day, since the passengers were in relatively sealed train cars with clean air a small distance from the fire, meaning there was no immediate risk of the blaze spreading to where the passengers were. The fire is reported to be extinguished by 10:20pm, having gutted a van on the upper deck of the car carrier and largely destroyed a second car while dealing severe damage to the car carrier. The overhead catenary had also failed and fell onto the train. The decision was made that the train was not going to be driven or towed out of the tunnel, but there was also still too much smoke to just have passengers evacuate on their own.
The firefighters thus started entering the train one car at a time, rescuing the passengers in small groups with the use of smoke hoods. Smoke hoods are folding, portable breathing masks fitted with an air filter and/or an attachment for oxygen bottles, allowing firefighters to rescue civilians from smoke-filled areas. It took the firefighters over an hour to evacuate all 150 passengers and the train crew through two emergency access points, handing them off to medical personell once reaching the surface. The train driver was later commended for staying on the phone with the emergency call center for over 50 minutes throughout the rescue, relaying important information back and forth.
Firefighters also searched the length of the tunnel once the train was confirmed to be empty, as they had initially been told there were 370 passengers. However, with no sign of passengers in the tunnel, it was eventually determined that that number were seat-reservations on the train, and not all passengers had boarded at the beginning of the train’s journey. A total of 33 people ended up being treated for minor smoke inhalation, with the rest of the passengers being taken to hotels in the area by bus.
Aftermath
The train was eventually towed out of the tunnel the next day, once ventilation had cleared enough smoke out of the tunnel for workers to safely recover the stranded train. It was taken to Innsbruck main station where passengers had their luggage returned to them and most of the undamaged cars were unloaded. The original suspicion of the cause laid with the catenary, suspecting that it had snapped and fallen onto the train, starting the fire.
The extremely high voltage running through the overhead catenary of a rail line is strong enough to cause “arching” (continuity of electrical flow without requiring physical contact) even when the other object is still a short distance away from the wire. This often underestimated capability regularly leads to tragic incidents when people climb onto parked trains suffering severe if not fatal electric shocks when they even only get close to the wire. A train’s pantograph (the articulated contact-rig used to connect most electric locomotives to the catenary’s power supply) does slide along the wire, but it’s actually enough to get near it to be hit with a high-voltage shock. It would be easy for a wire to snap, be it due to damage or a faulty pantograph, and, making contact with the cars on a train, lighting them on fire from the heat created by the current.
However, investigators at Innsbruck soon figured that the process wasn’t quite as straightforward in the case of the tunnel fire. Because a pantograph getting tangled in the wire and snapping it also damages the pantograph, and those atop the Nightjet’s locomotive were fine. The car carrier which caught fire was also the second car of the train, much too close to the locomotive for the train to come to a halt from a snapped catenary with it below the tear. And lastly, there was no sign of arching on the train ahead of the burned cars, which would have occurred if the locomotive had snapped the catenary and left it dragging along the train until it came to a stop.
Attention turned to the burned cars on the car carrier, mainly the van on the upper deck which had been reduced to a scorched shell on wheels. A first hint at the cause of the fire is the van’s roof, or rather its lack thereof. The steel roof hasn’t melted, and vans of the type don’t come with a glass sunroof that big. The answer is provided by a row of distinct drilled holes along the edge of the roof and some hinges at the back of the vehicle. The van had been a camper with a pop-up roof, which has just about entirely ceased to exist in the blaze.
The van was properly booked for the train, being within the height- and width-limits (if barely, regarding height), and was parked on the upper level of the car carrier close to the middle of the train car. The DDm-type car carriers’ upper decks are actually slightly curved downward towards the middle, allowing a few extra centimeters for tall cars on the upper deck. The position of the remaining hinges does suggest that the roof was at least partially up at the time of the fire. The van was most likely fitted with a hydraulic roof, which is operated similar to a car’s tailgate with hydraulic cylinders pushing the forward part upwards once the mechanism (commonly automatically after a button is pushed) unlatches.
The investigators find that a defect in the roof-mechanism likely caused the roof to unlatch itself unintentionally, at which point it got pushed up against the catenary. Most of the roof itself is made of fiberglass (explaining why it so completely disappeared in the blaze), but pieces of the hydraulic system, some reinforcements and the bolts holding it together are metal, and that was likely enough for the current to jump to the car, causing arching which, through the involved heat, set fire to the interior of the van. The fire sustained itself on the van and escalated by igniting the car below from the radiating heat, escalating the fire. The roof, before being consumed by the fire, most likely tore the overhead catenary off one of its supports (the alternative being that the arching caused a short circuit), causing the power failure which brought the train to a halt. The catenary then snapped just before the train came to a stop as the heat from the fire weakened it.
The ÖBB banned cars with any sort of pop-up roofs from the Nightjet-Services in the aftermath of the accident, reimbursing customers who had already booked with such cars. The introduction of the second generation Nightjet-trains eventually put an end to any car-carrying on the Innsbruck-Amsterdam-line by 2024, since the new trains are permanently coupled seven-car groups doing away with the car-carriers as part of the train.
The tunnel remained closed for two days, bottle-necking all traffic through the limited capacity of the old rail line, before limited operation on one track could restart as repairs finished up.
While there isn’t really a lesson to learn to avoid an accident like this it has to be noted that extensive training of the responders for this unusual scenario (train fire in a tunnel) likely limited the severetiy of the outcome for all involved. Fires in a tunnel with people involved are among the worst case scenarios a firefighter can encounter, and to this day time and again claim lives due to the specific conditions compared to fires above ground. There was no addressable cause on the side of the railway, no “fix” to avoid a repetition, making this one of the accidents where the only thing one can do is try and prepare the best possible rescue-effort.
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