Structural health monitoring in rail
Structural health monitoring (SHM) is the continuous or periodic measurement of physical parameters in the bridges, tunnels and embankments of European railway infrastructure, detecting deterioration before it affects safety or availability.
Railway civil infrastructure in Europe is characterised by age. Well over 300,000 railway bridges exist across European networks, as mapped by the FP6 Sustainable Bridges project (2003–2007), whose survey dataset of over 220,000 bridges owned by 17 railways found more than 35% already over 100 years old at that time — and 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 extends sensors and measurement to bridges, tunnels and earthworks, adding continuous data from permanently installed sensors, indicating 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 therefore 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 fibre. Multiple FBG sensors can be multiplexed along a single fibre, enabling quasi-distributed measurement at many discrete points.
FBG systems are immune to electromagnetic interference. In railway environments, where traction return currents are present in nearby conductors, this is a significant advantage.
Distributed fibre-optic sensing
Distributed Fiber Optic Sensing (DFOS) systems take this further: the entire fibre functions as a continuous sensor, with spatial resolution ranging from centimetre scale for Rayleigh-based systems down to around a metre for Brillouin- and Raman-based systems over longer distances.
Brillouin and Rayleigh scattering enable strain and temperature measurement over kilometre-length structures from a single interrogation unit, while Raman scattering is sensitive to temperature only and is often used alongside Brillouin systems to separate the two effects.
A single fibre 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 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 has been deployed on a masonry arch railway bridge in the UK, providing long-term records of structural behaviour and quantifying the effect of repair interventions, while DFOS has been used to monitor a masonry arch railway bridge in Italy under train-induced moving loads.
DFOS installations on steel and concrete bridge structures generate continuous strain distribution data that feeds structural assessment models.
Under the Shift2Rail programme, five technology demonstrators addressed proactive assessment, repair and upgrade of tunnels and bridges. The EU-Rail Joint Undertaking has indicated that inspection costs for this infrastructure could potentially be halved through improved inspection methods and techniques.
Challenges
The data volume from continuous SHM systems is large. Separating mechanically induced signals from temperature effects, seasonal variation and sensor drift requires robust analytical workflows.
Weigh-in-motion data extracted from fibre-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 EN 1990, supplemented by CEN/TS 17440 for the assessment of existing structures, and by individual infrastructure manager requirements.
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