The 100% Availability Challenge

Proven and Hardened Redundant Axle Counter Application with Az LM

The reliable detection of trains is fundamental to the safe operation of modern railways. This puts a great demand on the availability of modern train detection systems, because availability is a precondition for the reliable and safe detection of trains.

Since the reliability of any system is limited by the given MTBF values of the individual components within the system, only redundancy can improve the availability of the overall system. In areas with a high impact on operations, a 100%-availability is indispensable, e.g. on lines with a high traffic density, on operational bottlenecks or in tunnels, where the site access is limited or a train forced to come to a standstill causes major congestion.

The availability of a train detection system can drastically be increased by adding redundancy. The ultimate train detection system is fully duplicated to allow any component to fail but the signaling circuit remains functional. This arrangement provides highest availability, provided that both systems are of SIL-4 type and the failed systems can be restored within a reasonable period of time.

Where the indoor equipment already provides some redundancy (e.g. a 2-out-of-3 system), only the outdoor components need to be duplicated (including cabling) to achieve full redundancy. The redundant configuration of a single 2-out-of-2 system uses the complete duplication of indoor and outdoor components. This redundant system concept has been applied and successfully proven with Thales Axle Counter System Az LM in various projects.

Thales Az LM with duplicated systems have been in service since 2007 making Thales the frontrunner in this technology worldwide.


In Switzerland two lines with three major tunnels under the Alps are using a full redundant Az LM design: The Gotthard line with Gotthard and the Ceneri base tunnel and the Lötschberg line with the Lötschberg base tunnel.

Ceneri base tunnel: 22 km long, double track, in service since 2020, max: speed 275 km/h, max. 250 trains per day

Gotthard base tunnel: 57 km long, double track, the longest railway tunnel in the world, in service since 2016, max. speed 275 km/h, max. 250 trains per day.

The train detection system for the Gotthard base tunnel project in Switzerland is designed using a redundant fibre optic backbone to connect Detection Points, Axle Counter Evaluator and Interlockings.

Lötschberg base tunnel: 34 km long, double track, in service since 2007, max: speed 250 km/h with a headway of 180 seconds, max. 120 trains per day


In Netherlands, the double track high speed line HSL-Zuid with trains running at a maximum speed of 300 km/h creates a high demand on the availability of the system and has therefore been equipped also with a full redundant Az LM Axle Counter system.

HSL-Zuid high speed line

HSL-Zuid high speed line
147 km long, double track, in service since 2009, max. speed 300 km/h, 270 trains per day

Since non-availability of the track infrastructure very often causes compensation payments from the owner of the infrastructure to operations, the additional investment reduces the overall Life Cycle Cost and enhances safety by a 100% availability.

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Axle counter system Az LM

Thales' flagship axle counter system for train detection
Driven by the need to have a reliable train detection system with life cycle costs far below that of track circuits, axle counter systems are being deployed by an ever increasing number of infrastructure managers.

The increasing number of passengers on main line, suburban rail, metro and tramway as well as freight transport on main line, has fuelled the need for an axle counter system which ...Read more

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