Structural health monitoring
Structural health monitoring (SHM) in rail is the continuous or periodic measurement of physical parameters in bridges, tunnels and embankments, used to track structural condition over time and detect deterioration before it affects operational safety or availability.
Railway civil infrastructure in Europe is characterised by age. There are over 220,000 railway bridges across European networks, of which more than 35% are over 100 years old (European Commission Sixth Framework Programme, Sustainable Bridges project). Many carry axle loads and traffic frequencies significantly higher than their original design assumptions.
Conventional maintenance relies on periodic visual inspection supplemented by manual measurement. SHM augments this with continuous data from permanently installed sensors, providing the data needed for maintenance decisions: when to intervene, at which location and with what urgency.
Sensor technologies
Strain gauges are the baseline technology for structural monitoring. Bonded directly to structural elements, they measure local deformation as a change in electrical resistance. Each gauge covers a single measurement point; characterising a large structure requires an extensive sensor network.
Fiber Bragg Grating (FBG) sensors are optical devices that measure strain by detecting shifts in the reflected wavelength of light from a grating inscribed in an optical fiber. Multiple FBG sensors can be multiplexed along a single fiber, enabling quasi-distributed measurement at many discrete points.
FBG systems are immune to electromagnetic interference, a significant advantage in railway environments where traction return currents are present in nearby conductors.
Distributed Fiber Optic Sensing (DFOS) systems take this further: the entire fiber functions as a continuous sensor, with measurement points at centimetre-scale intervals over its full length.
Technologies based on Brillouin, Rayleigh and Raman scattering enable strain and temperature measurement over kilometre-length structures from a single interrogation unit. A single fiber installed on a railway bridge span can simultaneously monitor load distribution, detect localised damage and provide weigh-in-motion data as trains pass.
Accelerometers measure the dynamic structural response under traffic loading — modal frequencies, damping ratios and dynamic amplification factors that shift as structural condition changes.
Applications in Europe
Bridges are the primary focus, driven by asset age and the increasing axle loads associated with freight traffic growth. FBG monitoring systems have been deployed on masonry arch railway bridges in the UK and Italy, providing long-term records of structural behaviour and quantifying the effect of repair interventions.
DFOS installations on steel and concrete bridge structures generate continuous strain distribution data that feeds structural assessment models.
Under the Shift2Rail and EU-Rail programmes, bridge health monitoring was developed as a technology demonstrator; the EU-Rail Joint Undertaking has indicated that inspection costs for tunnels and bridges could potentially be halved through enhanced monitoring methods.
Challenges
The data volume from continuous SHM systems is large, and separating mechanically induced signals from temperature effects, seasonal variation and sensor drift requires robust analytical workflows. Weigh-in-motion data extracted from fiber optic sensors during train passages must account for speed and consist variation to yield calibrated axle load estimates.
There is no European standard specific to railway SHM of a scope comparable to EN 13848 for track geometry. Current practice is guided by the Eurocodes structural assessment framework and by individual infrastructure manager requirements.