Programmable electronic interlocking system plays key role in railway operation and is closely related to railway accidents, in which the programmable electronic controller of interlocking system may become sources. Redundant digital controllers are adopted as the interlocking controllers to prevent the accidents from the controllers being out of order. The redundant digital controllers being fault tolerant are realized through dual or triplex controllers. In this paper, we calculated safety and availability of the redundant digital controllers using Markov models, demonstrated key part to determine the availability ana the safety.