The aim of this research is to identify problems at transition zones, also known as critical zones, which are between an open track and a super structure, by means of field monitoring. Data has been collected using vehicle-borne instrumentation, including bogie-mounted inertial sensors. The purpose of this research is to simultaneously measure the vertical displacement of mam" consecutive sleepers by means of sleeper mounted Positional Sensitivity Device (PSD) sensors and geophones connected via a Controlled Area Network (CAN) bus, which enables data transmission among multiple nodes. This paper summarises the initial work carried out with a prototype of the PSD sensor. PSD sensors were tested in the laboratory using different electronic components and fine tuned with a range of central frequencies using a programmable oscillator. In future work the sensor network will be deployed, which can cover up to 16 sleepers in a known critical zone to obtain data during the train passage. In addition, to supplement the constraints of each methodology, comparison tests between the PSD sensor and geophone sensors will be part of future work. (5 pages)