Security communication systems composed of highly reliable components may have few if any failures while undergoing heavy testing or field-usage. This paper combines (i) analysis of randomized pulse modulation schemes based on finite Markov chains with (ii) security communication systems failure as a rare event with a finite-state, discrete-parameter, recurrent Markov chain that models the failures. An application of these results would be (i) in the analysis of randomized pulse width modulation implemented into the command of the railway data transmitter for the traffic security, and (ii) in the analysis of these system`s reliability.