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

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.

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.