Usually EMK or WMK.
Intermediate signal locations that use newer versions of ElectroCode (or equivalent Microtrax) can indicate a malfunction via the coded track circuit. For example, if power is lost at an intermediate signal location, it will stop transmitting Code M. A nearby control point will detect the loss of Code M and send an indication to the office. The office doesn't know exactly what is wrong or where, but it knows that something happened to an intermediate signal east or west of the CP.
-- Main.GaryHahn - 19 Jul 2008