--- title: Railway FBG Sensors: Axle Counting and Rail Strain | DuyuSense url: https://www.duyusense.com/en/solutions/railway-fbg-axle-counting --- [Home](https://www.duyusense.com/en/) / [Applications](https://www.duyusense.com/en/applications) / Railway FBG Sensors: Axle Counting and Rail Strain Railway FBG # How are FBG sensors used on railways? Axle counting and rail strain Short answer On railways, Fiber Bragg Grating (FBG) sensors are mounted on rails, pantographs, turnouts and bridges to measure strain and temperature at discrete points. From these readings come axle counts, wheel-flat detection, weigh-in-motion, wheel loads, pantograph contact force and rail buckling risk. The sensors are passive and metal-free, so traction power does not disturb them. DuyuSense pairs its patented FBG interrogation system with railway R&D and signaling experience. Last updated: 2026-09-30 · Prepared by: Dr. Serhat Boynukalın, DuyuSense ## How does an FBG axle counter work, and why does EMI immunity matter near traction power? An FBG axle counter works by reading the strain pulse each passing wheel creates in gratings bonded or clamped to the rail web, recorded as a wavelength shift. The time offset between two closely spaced sensors gives direction and speed, and comparing axle counts at the entry and exit of a section shows whether it is clear. On electrified lines, traction return current in the rails, harmonics from braking and lightning surges are well-known sources of interference for inductive counters that sense magnetic fields. FBG measures with light and needs no trackside electronics or power; the signal travels kilometres over fiber to an interrogator in a technical building, and many measurement points can share a single fiber. An axle counter is a safety-critical signaling component. Using an FBG axle counter for vital train detection depends on an independent SIL assessment under CENELEC **EN 50126** (RAMS), **EN 50128** (software) and **EN 50129** (safety case). Without that assessment, FBG serves as a complementary monitoring layer alongside safety-approved systems. - Immunity to traction current, harmonics and lightning-induced interference - Passive measurement points with no trackside electronics or power supply - Direction, speed and wheel load from the same sensor at the same time ## How do rail strain sensors detect wheel flats and weigh trains in motion? Weigh-in-motion (WIM) calculates the vertical load of each wheel by measuring shear strain in the rail web between two sleepers as the wheel passes. FBG sensors apply this classic layout in place of electrical strain gauges; near 1550 nm the wavelength shift is typically about 1.2 pm per microstrain. A wheel flat appears as a short, sharp impact riding on top of the normal load signal as the wheel crosses the measurement zone. Where several sensors are placed in sequence, the impact repeats at intervals matching the wheel circumference, which identifies the defective axle and wheel. A high sampling rate is essential so the impact peak is not missed. - Load per wheel, axle and wagon, plus total train weight - Detection of overloaded and unevenly loaded wagons - Wheel flats, out-of-roundness and wheel impact load ## How can FBG monitor wheel loads and derailment risk? FBG arrays placed at different points on the rail web measure vertical wheel force (Q) and lateral force (Y) together, producing the core indicators of derailment risk. Chief among them are the Y/Q ratio, lateral force divided by vertical force, and wheel unloading (ΔQ/Q). Continuous tracking of these ratios in curves, turnout zones and wind-exposed viaducts helps single out vehicles with bogie problems or uneven loading before they damage the track. The measurement is not a safety system on its own; it is an early-warning layer that feeds maintenance and operations decisions. - Y/Q ratio and wheel unloading (ΔQ/Q) monitoring - Lateral force and rail rollover tendency in curves - Early detection of vehicles with bogie or suspension faults ## How is pantograph-catenary interaction measured with fiber optic sensors? Pantograph contact force is determined by FBG sensors on the pantograph head and collector strip carriers that measure strain during contact with the wire. Because the fiber is fully dielectric, measurements can be taken at 25 kV AC or DC catenary voltage without extra isolation electronics. On the catenary side, FBG sensors can track tension in droppers, messenger and contact wires, contact wire uplift and temperature. Sudden drops in contact force point to arcing and loss of contact, while persistently high values point to accelerated wear; the data supports catenary adjustment and maintenance planning. - Dynamic pantograph contact force and loss-of-contact detection - Catenary wire tension, dropper forces and temperature - High-voltage measurement without galvanic isolation hardware ## How are turnouts, bridges and track transition zones monitored with FBG? At turnouts, FBG sensors measure contact between switch rail and stock rail, strain in the switch rail and crossing, crossing nose impacts and switch panel temperature. Rising impact load at the crossing is an early sign of wear and geometry degradation, and together with point machine condition data it feeds predictive maintenance. Bridge approaches and transitions from embankment to structure are where track stiffness changes abruptly and settlement accelerates. FBG sensors on rails, sleepers and the bridge deck track dynamic load amplification, differential settlement and thermal interaction between continuously welded rail and the bridge; rail expansion devices and bearings can share the same fiber line. - Switch rail contact, crossing impact load and switch panel temperature - Stiffness change and differential settlement in transition zones - Track-bridge interaction, expansion devices and bearings ## How is rail temperature, stress-free temperature and buckling risk monitored? Buckling risk in continuously welded rail is set by how far rail temperature rises above the stress-free (neutral) temperature at which the rail carries no axial force. In restrained steel rail each 1 °C of difference typically creates about 2.4-2.5 MPa of axial stress; compression on hot days drives buckling, tension on cold days raises broken-rail risk. In fully restrained rail, thermal expansion is blocked, so the stress does not show up directly as strain. The rail-bonded FBG is therefore read together with a free reference FBG at the same temperature; with FBG temperature sensitivity typically about 10 pm/°C near 1550 nm, the difference between the two gives the blocked thermal expansion, which is the axial stress. An absolute stress-free temperature needs a known reference state at installation; after that, drift caused by maintenance, curve breathing and track movement can be tracked. - Real-time rail temperature and axial thermal stress - Tracking of stress-free temperature drift from installation onward - Threshold alarms for hot-weather buckling and cold-weather rail breaks ## What does DuyuSense offer for railway FBG projects? DuyuSense is a fiber optic sensing company at ITU Teknopark, Istanbul, that develops its own FBG sensors and a patented FBG interrogation system built for real-time, high-speed operation in harsh environments. It also provides railway R&D engineering, signaling consultancy and railway energy efficiency analysis. Signaling experience makes it possible to settle early in a project where FBG measurement complements a safety-approved system and where it would require a SIL assessment. With founder Dr. Serhat Boynukalın's decade-plus industry experience and academic background in fiber optic sensing, DuyuSense covers the path from sensor design to a field-tested prototype and is open to pilots and project partnerships with operators and system integrators. - Custom FBG sensor design and prototyping for rails, pantographs and turnouts - Signaling consultancy and system integration - Combined design of line-wide DAS monitoring and point FBG measurement - Pilot design and training support **FBG axle counters compared with inductive axle counters and electrical strain gauges** | Feature | FBG (fiber optic) | Inductive axle counter | Electrical strain gauge | |---|---|---|---| | Measurement principle | Bragg wavelength shift from rail strain | Wheel flange disturbs a magnetic field | Electrical resistance change from rail strain | | Electromagnetic interference | Immune, passive and dielectric fiber | Suppressed by design, traction current needs countermeasures | Sensitive, needs shielding and grounding | | Trackside electronics | None, interrogator in a technical building | Detector electronics at the rail | Bridge circuit and amplifier at the rail | | Cabling | Many points on one fiber, long distances | Copper cable per detector | Copper cable per measurement point | | Information delivered | Axle count, direction, speed, wheel load, flats, temperature | Axle count, direction, speed | Wheel load, flats, weighing | | Safety approval | Product-specific, vital use needs a SIL assessment | SIL 4 approved products are common and mature | Usually monitoring only, not vital | ## Frequently asked questions ### Can FBG sensors replace inductive axle counters? Technically, FBG can perform axle counting without being affected by electromagnetic interference. But an axle counter is a safety-critical component, so vital use requires an independent SIL assessment under EN 50126, EN 50128 and EN 50129. Until that assessment is complete, FBG is used as a monitoring layer that complements safety-approved systems. ### Is SIL 4 possible for an FBG axle counter? A SIL level depends not on the sensing technology but on the whole system design, failure mode analysis, software development process and independent safety assessment. An FBG-based axle counter can target SIL 4, but that requires completing the CENELEC EN 50126, EN 50128 and EN 50129 processes in full and approval by an assessment body. ### How does fiber optic weigh-in-motion work? FBG sensors on the rail web between two sleepers measure the shear strain created as a wheel passes. A calibration factor converts this strain to wheel load, and wheel loads are summed into axle, wagon and train weights. The same signal also flags overloaded and unevenly loaded wagons. ### How does FBG detect wheel flats? A wheel with a flat produces a short, sharp impact on top of the normal load signal in rail-mounted FBG sensors. With sensors in sequence, the impact repeats at intervals matching the wheel circumference. Impact amplitude and position identify the defective axle and wheel and help rank maintenance priority. ### How is pantograph contact force measured with FBG? FBG sensors on the pantograph head and collector strip carriers measure strain during contact with the catenary, and this is converted to contact force. The fiber is dielectric, so no extra isolation is needed at high voltage. Sudden drops in contact force point to arcing risk, persistently high values to accelerated wear. ### Can FBG monitor rail stress-free temperature? Yes, under certain conditions. Reading a rail-bonded FBG together with a free reference FBG gives the blocked thermal expansion, which is the axial stress. An absolute stress-free temperature needs a known reference state at installation; after that, drift from maintenance and track movement, and the resulting buckling risk, can be monitored continuously. ### Should a railway use DAS or FBG? They solve different problems. Distributed Acoustic Sensing (DAS) turns trackside fiber into a continuous vibration sensor for train tracking, rockfall and intrusion detection. FBG suits discrete, quantitative measurements such as axle counting, wheel load, pantograph force and rail stress. Most projects design the two together. ### What does DuyuSense offer for railway FBG projects? DuyuSense develops its own FBG sensors and a patented FBG interrogation system, and provides railway R&D engineering and signaling consultancy. It offers custom sensor design, prototyping, system integration and training support. 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