The undercarriage is the most demanding installation location on a vehicle: at the axlebox housing, all vibration generated by wheel–rail interaction is transmitted directly to the equipment mounted there, compounded by water, salt, brake dust and ice. This is where speed sensors, bearing-temperature sensors and axle-grounding contacts are installed; under the underframe are battery boxes, shunts, HV-circuit fuse and junction boxes, magnetic track-brake coils, the ETCS antenna and a coupler with an electrical section.
The axle-speed signal is safety-related. ETCS odometry uses two independent measurement systems - rotation sensors on the wheelsets and Doppler radar; the same signal is used by wheel-slide protection, traction control, the event recorder and automatic sanding after wheel slip is detected. Vehicle grounding closes the return circuit: traction current returns to the rails through axle-grounding contacts on the axleboxes, while protective resistors of approximately 0.1 Ω protect the bearings against current flow; not every axlebox has space for a grounding contact because some positions are occupied by sensors.
Metro and tram vehicles without an overhead line collect current from a third rail - a collector mounted on the bogie maintains constant contact regardless of rail irregularities, while the contact shoe material (pure carbon, copper-impregnated carbon, lead-free material or a serviceable design) determines friction and the service life of the current rail itself. The wheel flange contacts the rail head during running; liquid lubrication creates a risk of loss of track-circuit shunting, so dry systems with a spring-loaded carbon stick are also installed under the underframe - without the risk of interfering with signalling.
Equipment used in this area includes: encoders and speed sensors for bogie axles (incremental, absolute and SIL-rated), encoder-signal splitters, speed-signal converters, axle-grounding devices and their contact elements, third-rail current collectors, HV-circuit fuse and junction boxes under the underframe, multipole connectors for couplers and inter-car connections, high-current terminal blocks, and wheel-flange lubrication systems. In our range, axle-grounding devices, HV fuse and junction boxes, third-rail current collectors with contact shoes, and wheel-flange lubrication systems are supplied by a single manufacturer - Mersen.
