A broken wheel disc was the cause of the derailment of a freight train in the Gotthard Base Tunnel on August 10 2023, the damage from which resulted in a near 13-month closure of the world’s longest railway tunnel, according to the final report on the incident published by the Swiss Transportation Safety Investigation Board (Sust) on June 2.

Sust describes the cracks on the affected freight train as “systematic phenomenon.” Based on its findings, and to prevent similar incidents from occurring in the future, Sust is issuing several safety recommendations to the European Union Agency for Railways (ERA) and the Swiss Federal Office of Transport (FOT).

According to the report, on August 10 2023, SBB Cargo train 45016 travelling from Chiasso to Basle and consisting of two locomotives and 30 wagons of different types, derailed 10km into the western bore of Gotthard Base Tunnel. A fragment of the right wheel disc of the first axle of the 11th wagon broke off, with further fractures of wheel disc following, causing the axle to derail. The train continued for approximately 7km to the Faido multi-function station where the switch drives on a turnout were destroyed.

The front section of the train passed through the turnout. However, the rear section was diverted on to the connecting track towards the eastern bore. This led to the derailment and collision with the tunnel cross wall. Sust says the damage to the rolling stock and infrastructure was considerable. No-one was injured in the accident.

Sust found that thermal overload of the running surface led to the fatigue cracks, which developed within the wheel disc until it fractured. The wheel was equipped with composite LL brake blocks, which the report says are favoured due to reduced noise emissions, but have been found to increase thermal stress on the wheel, with no adjustments made to maintenance procedures to take this into account.

It adds that the design of the turnout at Faido multi-function junction also contributed to the damage. Sust says that the control and locking elements of the Hydrostar turnouts are located above the sleeper and were destroyed by the axle of wagon 11, which was hanging transversely beneath the wagon. This caused the point to enter an undefined position, causing wagon 14 to derail and all subsequent wagons to be diverted onto the connecting track towards the tunnel.

The report shows a simulation of the position of the wagons from the back of the train after it had come to a halt.
Image: Swiss Transportation Safety Investigation Board.
Safety recommendations

Sust issues four safety recommendations and one safety notice in the report. Three of the safety recommendations are directed at ERA:

  • the existing regulations for increasing the currently applicable minimum wheel diameter should be extended to all wheelset types with composite brake blocks
  • the maintenance criteria for these wheelsets should be adjusted, particularly regarding maintenance intervals and testing methods, and
  • a study should investigate the influence of composite brake blocks on the thermal load on the wheels.

The fourth safety recommendation, concerning the location of the point locking devices, is addressed to FOT, and recommends assessing the risk of mechanical damage to these devices and reducing it if necessary. The safety notice, addressed to SBB Cargo, concerns the documentation of technical inspections of trains.

SBB response

Swiss Federal Railways (SBB) says it supports the recommendations made by Sust and calls for rapid implementation.

SBB says that until official measures for modernisation and maintenance have been adopted, it will gradually phase out the transport of freight wagons fitted with LL brake blocks, favouring composite K brake blocks. The majority of the phase-out will take place by the end of 2025, SBB says. In addition, it will inspect the freight wagons more intensively to detect visible defects, although pointing out that the detection of hairline cracks in wheels caused by thermal overload is only possible in maintenance depots.

LL-brake blocks are the most widely used in Europe although SBB says it favours alternatives due to “numerous incidents” in which LL brake blocks have led to overheating of the wheel discs along with ice formation, which can damage the wheel discs.

SBB says the wagons, which it transports across Europe, are owned by a variety of international owners. Yet, while bearing the risk and financial consequences of derailments, it has no input on manufacturing and maintenance of rolling stock.

“The accumulation of events in Europe shows that the risk of accidents has increased,” SBB says in a statement. “LL brake blocks lead to higher thermal loads on wheels than the earlier cast iron brake blocks. Nevertheless, according to the STSB report, the same maintenance methods are still used. Thermal loads can lead to stresses and cracks, which at best cause wheel breakages with potentially devastating effects.”

SBB adds that “effective measures are therefore needed as quickly as possible” and calls on FOT and ERA to act quickly. It confirmed that it has already implemented several measures in connection with the STSB recommendations, including reducing speeds at the portal areas of the Gotthard Base Tunnel to 160km/h. It is also planning to install derailment detectors at turnouts outside of and within the tunnel, with a prototype system currently under development.