Every railway vehicle is fitted with a complete compressed air system (including a compressor and a dryer). Compressed air is used for activating the brake system and the door opener. If there is excess moisture in the air lines, then there is a risk of condensation. This can lead to:
Monitoring the dew point at the dryer output and at other key points throughout the compressed air system means that any failure of the dryer or moisture ingress at any point can be quickly detected and acted upon by maintenance staff.
This is part of an integral maintenance procedure for all rail vehicles. Ensuring that the air in the system is sufficiently dry avoids costs caused by damage and repairs to parts which have been adversely affected by moisture. It also means that the vehicle will be less likely to succumb to sporadic failures, which would result in a loss of operating time, and perhaps safety issues. Failure of the brake system due to it freezing up would be a serious hazard.