A two-level alarm monitor for use with a hot box detector system of the type having first and second sensors each associated with a rail of a track is provided. The monitor includes means connected to the outputs of the sensors for obtaining the difference between the output signals of the sensors and for comparing the difference alternately to first and second predetermined levels which may, for example, represent ''''warm'''' and ''''hot'''' differential alarm levels or differential alarm levels for friction and roller bearings. The outputs of the sensors are also alternately compared with third and fourth predetermined levels which could, for example, represent ''''warm'''' and ''''hot'''' absolute alarm levels or absolute alarm levels for friction or roller bearings. The output signals from the comparators are demultiplexed and utilized to generate an alarm signal in the event differential signals or absolute alarm signals exceed associated predetermined levels.