This recommended practice defines the functions, performance, interface, and environmental conditions of vital computers used in rail transportation systems such as railways, metros, light transport, and trams. This recommended practice is applicable to vital computers for onboard and wayside equipment. This recommended practice is not applicable to the application systems which are developed based on vital computers and does not define specific safety targets for application systems, but the safety-related conditions for application systems are proposed. It is difficult to regulate internal structure and technical realization of vital computers, because of the great diversity that exists. In order to encourage technical competition and prompt the development of technology and industry, this recommended practice does not constrain or recommend the internal structure and technical realization of vital computers. Only the preliminary interface is defined in this recommended practice, on a function level. The detailed interface of a vital computer is not covered and can be defined in the form of external interface specifications. Compliance with the provisions of this recommended practice does not guarantee or ensure safety, security,health, or environmental protection. Users are responsible for observing all applicable legal and regulatory requirements.