Background
Olten is a city of 18496 people (as of December 2020) in northern Switzerland, located in the Canton of Solothurn 32km/20mi southeast of Basel and 47km/29mi west of Zürich (both measurements in linear distance).
Olten station is one of Switzerland’s busiest train stations, connecting half a dozen rail lines along with holding one of the main SBB (Swiss national railway) maintenance facilities and a shunting yard. Taking the amount of trains stopping at the station per day into account along with daily passenger numbers Olten station becomes the second-most important station in Switzerland after Zürich main station. All but three SBB Intercity services stop at the station, along with various regional passenger services, and even a few international (Eurocity) services. In addition the station also sees six connections by the Basel S-Bahn, an urban and suburban commuter rail system.
The trains involved
Train number 17075 was an express regional service (Interregio) from Basel to Olten, consisting of an SBB Re 420 and four second generation SBB “Einheitswagen” (referred to as EW II), a standardized four-axle passenger car. The Series Re 420, originally called the Re 4/4 II before computerization of the numbers, is a four-axle multipurpose electric locomotive introduced in 1967. It’s the backbone of the Swiss railway network, pulling everything from freight trains and regional services to long distance express trains. The locomotives measure 14.9m/49ft in length at a weight of 80 metric tons and can reach a top speed of 140kph/87mph thanks to a power-output of 3700kW/4962hp. Pulling the train at the time of the accident was Re 420 number 11184.
Also approaching Olten station from the north was S-Bahn service number 9 (S9) from Läufelfingen to Olten, provided by an SBB RBDe 561. The series 561, originally called RBDe 4/4, is a four-car electric multiple unit consisting of a motor car, (usually) two adapted EW I or EW II passenger cars and a cab car at the other end. Introduced in 1987 the RBDe 4/4 brought a standardization and modernization to regional passsenger services in Switzerland, hence the project-name “Neuer Pendelzug” (NPZ, “New push-pull train”). Each motor-car measures 25m/82ft in length at a weight of 70 metric tons and has a power-output of 1680kW/2253hp allowing the series 561 a top speed of 140kph/87mph. At the time of the accident the NPZ involved in the accident ran in a shortened configuration (no middle cars) and with the cab-car (Bt 29–35976–4) leading and the motor car (561 171–0) pushing from behind.
The accident
On the 6th of October 2011 the Interregio from Basel to Olten is approaching Olten station at approximately 5:45pm. It’s scheduled to use track 11 at the station, but due to the preceding regional express service from Luzern being delayed the usual track is not available. The dispatcher thus sets a path for the Interregio to use track 3, which also makes it easier to send the train on its return trip after the stop (scheduled for 5:54pm). Setting the path for the Interregio means the S9 (arrival scheduled for 5:53pm) has to hold outside the station. The section-signal B161 is set to red, along with the pre-signal B151, pre-signal A*151 and the entrance-signal A151 being set to “expect stop”.
The S-Bahn service in question has just reached Trimbach station, the last stop ahead of Olten. Due to the proximity of the stations (Trimbach station is just under 1.3km/0.8mi from the site of the accident) Olten’s pre-signal A*151 is actually located ahead of Trimbach station, where it instructed the driver to expect a red signal ahead of Olten station itself. To avoid accidents caused by train drivers forgetting the setting of A*151 (something that has happened several times, and has been featured on this blog before) a repeat-signal (labeled A**151) is located behind Trimbach station, reminding drivers of what A*151 showed. Pulling out of Trimbach station at approximately 5:49pm the S9 accelerates to 68kph/42mph as the train passes the repeat-signal, more than allowed, which triggers a warning in the cab.
Despite the warning the train picked up more speed regardless, according to the report largely due to the downhill path of the rail line not being met with an application of the brakes, reaching 77.8kph/48.3mph before being slowed to 72kph/44.7mph as it passed the entrance-signal A151. The same mast also carries pre-signal B*151, which indicated that section-signal B161 is red. The driver of S9 confirmed this information and slowed the train to 56kph/34.8mph before easing off the brake (leading to the train picking up speed again as it travelled downhill) and then applying it again, a pattern repeated once more leading to the S9 passing red section-signal B161 at 56.4kph/34.8mph instead of stopping at it. Running the red signal triggered an emergency stop at 5:51pm, but there simply wasn’t enough space between the signal and the set of points merging the S9’s path with that for the Interregio.
The two trains came together exactly on the set of points, with the 34kph/21mph S9 striking the forward right hand side of Re 420 11184, which was still travelling at 84kph/52.2mph. The S9 managed to push the Interregio off the tracks into the gravel ballast, scraping past the rapidly slowing train and causing damage to both trains. The locomotive of the Interregio suffered extensive damage to its cab from the initial impact with the S9 along with subsequently running over two overhead catenary support masts. The trains came to a stop a short distance apart, with the S9 derailing but staying somewhat aligned with the tracks while the Interregio’s locomotive and first two cars completely derailed and ended up leaning to the left at up to 50 degrees. The driver of the Interregio suffered severe injuries and one of his passengers was minorly injured, no one aboard the S9 suffered any injuries.
Aftermath
Investigators quickly figured out that both trains were in perfect order ahead of the accident, and there was no sign of a defect in the signal box or signaling system either. Setting an “approach with conflict potential”, meaning only one signal kept trains apart ahead of merging tracks, in itself was also nothing out of the ordinary. Sometimes such intersections use protective points, sets of points that automatically derail the train approaching without permission before it reaches the line its track is merging into, but that isn’t required.
Interviewing the driver of the Interregio and examining the Re 420’s datalogger showed that the driver had done literally nothing wrong, he had even been driving defensively and stayed slightly below the speed limit on the approach into Olten. He didn’t get to trigger an emergency stop ahead of the accident, but in the mere seconds he might have seen the other train prior to the collision, if at all, it also would have had no effect.
As it soon became clear the fault for the accident rested solely on the driver of the S9, who had acted negligently on approach into Olten station. It is likely that he had trusted in routine, which led him to act in false certainty that he had a clear path into Olten station after departing Trimbach. He didn’t usually have to let the Interregio past, so, in simplified terms, in wanting to reach Olten faster he might have not taken the signals serious/might not have paid sufficient attention to them.
At the time of the accident the S9’s rail line was secured with Integra-Signum, the standard train protection system for Swiss regular gauge railways introduced back in 1933. The system was developed after a 1924 train collision at Bellinzona claimed 15 lives when a train ran a red signal. Integra-Signum warns a driver if they approach a red signal or violate the speed limit, if the driver doesn’t push a button to acknowledge the warnings and/or runs a red signal an automatic stop is triggered. Trains and signals communicate via electromagnetic impulses of certain polarity being sent and received.
Usually train protection systems like Integra-Signum are meant to eliminate the “single point of failure”, following the concept that one single factor in rail operation can fail without leading to a catastrophic accident. Before Integra-Signum’s introduction most trains in Switzerland were staffed by a driver and a driver’s assistant, if the driver made a mistake (single point of failure) the assistant could point it out/correct him. Integra-Signum somewhat replaced the assistant’s role, with the bonus that rank-hierarchy can’t keep a mistake from being corrected. It also means that the “backup” to human error can’t suffer human error. The problem was that, since 1933, technology had advanced leaps and bounds, and Integra-Signum was seen as barely sufficient at best. More modern train protection systems had more capabilities, they could observe and control a train’s speed rather than just requiring the driver to acknowledge speed limits, for example.
Adding insult to injury was the fact that the rail line used by the S9 on the day of the accident was fitted with the more advanced ZUB train protection system which could have controlled the train’s speed remotely, but the system had not been activated yet.
Lastly, the report notes one more major failure to keep operations safe. The signal-positions at Olten station hadn’t been examined or changed in decades, leading to the section-signal B161 being just 65m/213ft from the points its meant to protect. A car might stop in that distance, even a Semi can shed 100kph/62mph in less than that, but a train simply can’t. Integra-Signum correctly triggered an emergency stop as the S9 ran the red signal, but by that point there was no way the train would stop in time. In usual operation the train would have slid into the other rail line and that would have been that, an incident that requires investigation, but no more. But as the dispatcher had set up the “approach with conflict potential” there wasn’t just an unauthorized “signal passed at danger” (“SPAD”) to handle, there was a train crash. The report notes that the position of B161 is in accordance with the relevant guideline, but that just means the guideline is outdated, calculating with slower speeds and thus much shorter braking-distances.
In late November 2011 the ZUB-system went online, replacing the old Integra-Signum system which was phased out in all of Switzerland by 2018. If the events that led to the accident would reoccur today the S9 would be unable to break the speed limit the way it did, and either be auto-stopped before it even reaches B161 or stop within the “overrun space” past it, ahead of merging into the other, potentially occupied rail line.
The SBB modernized a fleet of RBDe 4/4-units between 2008 and 2013, creating the “Domino”-series. However, the RBDe 561 is not included in the modernization-program and, while most outlived the unit involved in the accident (which was scrapped after the investigation) the SBB has since either sold or scrapped all units of the type. The Re 420 is also largely retired, with over 60 units having so far been scrapped after being retired (as opposed to those being scrapped due to damage). Thirty units remain in service with Zürich’s S-Bahn and a few more have found a home with other rail service providers as new locomotives and multiple units gradually take over the 420’s jobs.
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