Hi Chris,
the following information can be found in the canadian trackside guide 2014. 
Main track begins Sign: black letters T over M symbol on reflective  silver square. Located at the actual point where the main track begins.
Main Track Ends sign: black letters T over M symbol with black diagonal bar on reflective yellow square. Located at the actual point where the main track ends. Note: these signs need not be placed where the main track begins or ends a a switch.
hope this helps some,
regards,
jordan