Overview
ISO/TR 8955:2025 - Railway infrastructure - Track quality evaluation - Chord-based method is a Technical Report that explains how chord-based measurement systems relate to inertial measurement systems for assessing railway track geometry. The document focuses on the conversion and signal-processing relationships when applying the chord-based method with different chord lengths and chord divisions. It is applicable to mainline track gauges of 1 435 mm and wider and explicitly excludes urban/light rail, tramways and gauges narrower than 1 435 mm. This TR is intended to be used in conjunction with ISO 23054-1.
Key technical topics and requirements
- Measurement system relationship: Describes the two primary measuring systems - inertial measurement systems and chord measurement systems - and how they map to two evaluation approaches (wavelength range method and chord-based method).
- Transfer functions: Defines the colouring, decolouring (Wiener inverse), and recolouring transfer functions linking measured signals to evaluation signals; includes treatment of impulse/transfer functions in Fourier, Laplace and Z domains.
- Chord parameters: Discusses effects of chord length, chord division ratio, and normalized chord length on the transfer function and measurement outcome. Introduces chord length classes (short/long - C1/C2) used in practice.
- Colouring and decolouring processes: Specifies methods to generate a chord measurement signal from non-coloured signals (colouring) and to remove chord-induced distortion (decolouring). Techniques covered include digital filtering (FIR and IIR), space-domain and frequency-domain methods.
- Recolouring and verification: Presents methods to transform one coloured signal to another and verification approaches using test signals and recorded track geometry data.
- Practical considerations: Includes procedures for eliminating influence of track geometry layout, use of restored waveforms, and examples (Annex A shows a colouring example in MATLAB; Annex B provides verification guidance).
Applications and target users
ISO/TR 8955:2025 is valuable for:
- Track engineers and maintenance planners assessing longitudinal level and alignment.
- Manufacturers of measurement systems (inertial and chord-based) for calibration and compatibility.
- Data analysts and signal-processing engineers implementing colouring/decolouring and verification workflows.
- Rail infrastructure owners and procurement teams specifying measurement performance and interoperability requirements.
Practical uses include converting measurement data between systems, defining processing chains for track quality evaluation, and ensuring consistent track geometry metrics for maintenance decision-making.
Related standards
- ISO 23054-1 - Track geometry quality: Part 1 defines wavelength range methods and complements the chord-based approach described in ISO/TR 8955:2025.