Report on faulty interlocking system at Jar metro station on the Kolsås line on 27 November 2021
Railway report 2022/08
On 27 november 2021 Sporveien T-banen's traffic controller observed that a tram was not detected by the signalling system on one of the track sections at Jar station. The same condition was repeated with the next trains that passed. Lack of train detection is a dangerous condition because it enables following trains to receive a full speed signal onto a section of track that might be occupied by another train. This increases the risk of collisions. The traffic controller started to manually control the train movements until a signal engineer had searched for faults and corrected the error.
Sporveien-T-banen's own investigations on the day of the incident showed that a relay in the interlocking system was stuck in the closed position, thus giving the safety system wrong side information about the condition of the track circuit.
This relay type is used in a large number of signalling systems throughout the world. The construction is from the mid-1950s, and is considered very robust and reliable if used correct and properly mainteined. Errors of this type are very rare. The relay in question had recently arrived from the manufacturer Siemens Mobility Ltd. in England, and had been installed in the interlocking system for less than six months.
The NSIA has been unable to come to a definitive conclusion as to why the relay did not open when the track circuit was short-circuited by the vehicles, but it is likely that vane rotation was prevented by small metal fragments found in the bearings of the relay’s vane.
It has not been possible to determine how the metal fragments entered the bearings. The relay is a sealed unit that is only opened at the factory. A possible explanation comes from the way these bearings were manufactured or stored at the factory, or from improper cleaning during reassembly after being overhauled.
Siemens Mobility Ltd. has changed its assembly process after the incident. The pivot bearings are now cleaned with ultrasonic cleaners before assembly.
The NSIA does not issue any safety recommendations following the investigation.
VT1-6F-4B relay. Photo: NSIA
The fragmets found on the pivot ends. Photo: FOLAT
Pivot end, bore and fragment. Sketch: NSIA
Vane. Photo: NSIA
Published 16.12.2022
Facts
| Location | Jar metro station |
| Occurrence date | 27.11.2021 |
| Train No | 1316 |
| Type of Transportation | Metro |
| Type of occurrence | Wrong side signaling |
| Rolling Stock | Passenger Carriage |