A. Fixed Monitoring
Many authors have investigated health monitoring of infrastructure,
including rail bridges, tunnels, rail tracks and track
beds, and other track infrastructure. Krüger et al. [70] noted
that structural health monitoring is becoming ever more important
as both train speeds and the axle loads exerted by trains
increase. Until recently, inspection has been performed visually
[70], but this only examined the structure superficially and
intermittently. Additionally, the visual analysis needs to be interpreted
by an expert, which can be subjective. Sensors are objective
and can provide data from all of the structure (including
internally) to allow the whole structure’s health to be assessed
and to analyze its durability and remaining life time. Wired
sensors can be used for monitoring. However, wired systems are
expensive, inflexible, time consuming to install, and the trains
have to be stopped during installation [70]. The early fixed
WSNs were simple such as attaching sensor nodes to the rails
to monitor the rail temperature or low-voltage warning sensors
that monitor the power supply to motors at points [15]. These
systems simply generated a binary output (high or low). For the
temperature sensor, a rail expert with knowledge of the ambient
(stress free) rail temperatures set the alarm levels for “amber”
and “red” warnings. If an alarm level was reached, then the
system sent an SMS alert. Modern WSNs provide semiautomatic
or automatic analysis of the sensor data to examine
structural changes and to improve the durability of structures.
Hence, WSN monitoring should reduce the overall maintenance
costs. An overview of fixed monitoring systems is given in
Table II in the Appendix.