This paper presents the hazard analysis of a fail-safe computer system for railway applications called Simis PC, which is based on commercial off-the-shelf (COTS) hardware and software components. To meet the safety requirements of CENELEC safety integrity level 4, a fail-safe computer is used for voting the results of the multicomputer system. The hazard analysis proves that the safety measures implemented are sufficient to prevent systematic faults and detect failures. The fail-safe Simis PC computer system was first used in an electronic interlocking system for mass transit in Vienna in September 2003. It is now firmly included in the portfolio for mass transit, as well as main-line interlocking systems, for Europe, Asia and North America.