What failed technically in the train crash in Toledo?

BarcelonaBroadly speaking, railway safety relies on three technical layers that are often confused: interlocking, signaling, and protection. After the accident in Urda, Toledo, it is worth explaining them.

The block system divides the track into sections, called blocks, and only one train can be in each block. The interlocking is the brain of each station: it prevents a signal from opening in green if the switches are not properly set or if the route conflicts with another, just as an urban traffic light does not give way simultaneously to two crossing streets. Finally, the Automatic Train Protection (ATP) is an on-board system that monitors the train driver and brakes the train if it passes a signal or exceeds the allowed speed. The block system decides which signals open; the ATP checks that someone obeys them. The so-called Automatic Train Operation (ATO) does not protect but rather drives: it accelerates and brakes between stations like an automatic gearbox, but is always supervised by an ATP.

AVE, Rodalies, FGC and the subway

On the Adif network, the state rail infrastructure manager, systems of various generations coexist. ASFA (Anuncio de Señales y Frenada Automática - Automatic Signal Announcement and Braking) is a Spanish system that acts as a backup throughout almost the entire network. Emitter beacons placed between the rails transmit to the train what the signal indicates; if the train driver does not confirm having seen the warning, the emergency brake is activated. It only monitors at specific points, because it does not watch anything between one beacon and the next.

In contrast, the LZB (Linienzugbeeinflussung) supervises continuously thanks to a cable laid between the rails that communicates the control center with the train via magnetic induction; today it is only present on the Madrid-Seville AVE (where it seems to have failed on Wednesday) and on the C-5 line in Madrid. The current European standard is called ERTMS (European Rail Traffic Management System) and has two levels in service: at level 1, beacons provide a movement authorization each time the train passes over them; at level 2, communication is continuous, via the GSM-R railway radio.

In Rodalies de Catalunya, no line yet operates with ERTMS in commercial service: they function mostly with the conventional system: side signals, automatic block signaling, interlockings (in the process of being renewed for electronic models), and ASFA as protection. The first line to adopt ERTMS will be the R1 between L'Hospitalet and Mataró, precisely the one that will be assumed by the brand-new joint venture company Generalitat/Renfe.

Ferrocarrils de la Generalitat (FGC) follows a different path. On the Barcelona-Vallès line, an ATP system based on speed codes monitors the train's movement, and since December 2022, ATO has been operating there, with a driver in the cab. The Llobregat-Anoia line, less modern, has its own system, but FGC is preparing the transition to CBTC (Communications-Based Train Control), the same technology as the metro, based on radio. The AESF only supervises FGC trains when they circulate on the Adif network, but the Catalan railway safety agency foreseen in the Railway Strategy of Catalonia does not yet exist.

In the Barcelona metro, lines L1, L2, L3, and L5 combine ATP and ATO with a driver. The L4 is the only one operated entirely manually, only with ATP. Lines L9, L10, and L11 operate without a driver using the aforementioned CBTC. This system is closed and proprietary to each manufacturer, unlike the ERTMS, which is interoperable and allows trains from different operators to circulate on the same network.

When the main system fails

The weak point is not the variety of systems, but the occasional breakdowns: when the main system fails, the train runs in the so-called degraded mode, with a less demanding backup system and more human intervention. On the Adif network, if the LZB fails, as it seems to have happened on Wednesday, the section switches to lateral signaling blocking, with signals on the track and ASFA as on-board protection. Even older is the telephone block, where the officials of two consecutive stations "grant" the track to each other by phone; it works (until not long ago, on our R3 line), but safety depends on people's discipline.

A delicate moment is the transition between sections with different systems. High-speed trains carry equipment to read ASFA, LZB, and ERTMS, and when moving from one to the other, the driver must acknowledge it and the level of protection may drop. The best-known case is that of Angrois: on July 24, 2013, an Alvia derailed while traveling at 179 km/h on a curve limited to 80 km/h; 80 people died there. The final section was in an ASFA zone and the on-board ERTMS was disconnected. The Railway Accident Investigation Commission (CIAF) concluded that the ERTMS would not have limited the entry speed to the curve, because the limit that the system was monitoring at that point was 200 km/h; the 80 km/h limit of the curve only appeared on a sign.

In the Toledo case, the investigation is just beginning. Around 8:30 a.m. on Wednesday, according to the ministry, the LZB section control system went down due to a communication failure and the trains stopped. Adif switched to lateral signaling blocking with ASFA as a backup system, but the Atocha control center authorized – in a decision that Minister Puente described as "inexplicable" – the resumption of normal circulation, even though the procedure limited the speed to 40 km/h. The Renfe AVE collided from behind with an Iryo train stopped in the same section: the provisional count of fatalities is already bordering on 70. The Independent Administrative Authority for the Technical Investigation of Accidents (AITAT), which since July has replaced the CIAF, considers it too early to put forward hypotheses.

On the other hand, an anecdote illustrates the physical vulnerability of these systems: as a spokesperson for Prysmian, a cable supply company, explained to 3Cat, many of the most recent copper thefts in the rail network specifically affect signaling and control cables, which operate at low voltage and are easier to steal than the high-voltage catenary ones.

It should be noted that not all recent accidents have to do with protection systems, which do not detect a broken rail or the collapse of a wall. In Adamuz (Cordoba, on the same Madrid-Seville AVE line as last Wednesday's crash), on January 18, an Iryo train derailed and entered the adjacent track, where an Alvia train was circulating: 46 people died there. According to investigators, the rail broke before the Iryo passed. In Gelida, on January 20, an AP-7 wall collapsed onto the R4 track and killed a trainee train driver.

Regulations are coming at their own pace

The general calendar is set by Brussels: a European regulation mandates having ERTMS on the core network by 2030 and retiring national systems like ASFA between 2040 and 2050. On the Madrid-Seville line, the scene of the last two major disasters, the level 2 ERTMS was awarded to Thales in September 2021 with a deadline that expired at the beginning of this year, but we are already in October and it is still not the primary system. By the time it arrives, it will already be too late.