Broken wheel caused Gotthard Base Tunnel derailment, report confirms

On August 10 a freight train came off the rails, causing the temporary closure of both Gotthard rail tunnels and considerable damage. © Keystone / Urs Flueeler

The Swiss Transportation Safety Investigation Board (STSB) has confirmed that the derailment of a freight train in the Gotthard Base Tunnel last month was caused by a broken wheel.

The existing train control systems were unable to detect earlier damage to the wheel.

The broken wheel was caused by fatigue cracks in the metal, the STSB wrote in an interim report published on Thursday. Such fatigue cracks grow with continued use until the wheel breaks. The STSB found no evidence of operational defects as a cause of the accident.

According to the current train control systems used on Swiss railway infrastructure, the cracks could not have been discovered. STSB said a crack can only be discovered to a limited extent during technical tensile tests, and they also depend on its size. In addition, cracks can only be found in the visible area of ​​the wheel.

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The damage pattern corresponds to a safety warning by the Belgian and Italian supervisory authorities from 2017, STSB said. In 2016 and 2017, several cracks and breaks occurred in similar freight train wheels in Belgium and Italy. Measures were then taken to limit risks during operation and maintenance.

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The freight wagon that caused the derailment in the Gotthard Base Tunnel on August 10 belonged to a Zug company. It was the eleventh wagon being pulled by the train. According to STSB, a fragment of the wheel disc broke off about ten kilometres after entering the tunnel. More fragments then came loose.

The axle hung diagonally under the car. The last fragment broke off 17 kilometres into the 57.1-km tunnel. The axle hit and destroyed a switch. The following 16 freight cars then derailed. The train was torn apart between the 13th and 14th wagons and certain wagons ended up on the opposite track.

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