Effect of unsupported sleepers on wheel impact load detector performance

Paper in proceeding, 2026

Wheel–rail impact loads caused by discrete wheel tread irregularities, such as wheel
flats, can lead to severe damage to both vehicles and track infrastructure. To monitor
wheel out-of-roundness, peak loads are commonly measured using wheel impact
load detectors. Depending on the detector type, these peak loads may, for example, be
derived from post-processing of recorded rail shear forces, rail seat loads, or rail rotations.
To ensure accurate load measurements, minimise operational costs, and avoid
unnecessary stopping of trains, the detectors must be robust and reliable. This numerical
study investigates how unsupported (voided) sleepers within the detector area
influence the resulting wheel–rail contact force signature and the unprocessed (raw)
sensor signals in the presence of a wheel flat. The analysis considers void depths of
up to 4 mm beneath two adjacent sleepers. The effect of unsupported sleepers is analysed
through numerical simulations using the in-house software DIFF, in which the
dynamic vehicle–track interaction is solved in the time domain. The results show that
rail rotation and rail seat load are the most affected by unsupported sleepers, as the
increased local track flexibility alters the sleeper–ballast interaction and influences the
load distribution along the detector.