STM. Specific interface module, in the ERTMS system, able to translate the information received from various ground systems, different for each European network, in a format that can be understood by the same European ERTMS standard system. Therefore the STM units represent the function of connection between the different existing systems and the ERTMS standard. The same train can have multiple STM modules, how many are present in the lines it runs
Link... for your business