This part of EN 50126

• is intended to apply to all safety-related software aimed at electronic railway systems/sub-system. The relevant methods are provided by EN 50126-2. If analysis reveals that no safety requirements exist (i.e. the situation is non-safety-related), and provided the conclusion is not revised as a consequence of later changes, this part of EN 50126 ceases to be applicable;

• specifies the process and technical requirements for the development of software for programmable electronic systems for use in railway monitoring, control and protection applications. These systems can be implemented using dedicated microprocessors, programmable logic controllers, multiprocessor distributed systems, larger scale central processor systems or other architectures.

• is applicable exclusively to software and the interaction between software and the system/sub-system of which it is part.

This European Standard

• is primarily applicable to software which have been specifically designed and developed for railway applications. It should also be applied, as far as reasonably practicable, to general-purpose or industrial software which is procured for use as part of a safety-related railway system. As a minimum, evidence shall be provided in such cases to demonstrate:

- either that the software is not relied on for safety,

- or that the software can be relied on for those functions which relate to safety;

• applies

- to the specification, architecture, design, development, implementation, integration, installation, acceptance, deployment, operation, maintenance and modification/extension phases of the software in a system /subsystem. It also applies to individual sub-systems within the overall system as determined by the process in EN 50126-1 and supported by the methods in EN 50126-2,

- to generic sub-systems (both application-independent and those intended for a particular class of application), and also to systems/sub-systems for specific applications;

• does not define

- RAMS targets, quantities, requirements or solutions for specific railway applications

- rules or processes pertaining to the certification of railway products against the requirements of this European Standard

- an approval process by the safety authority;

• does not specify requirements for ensuring system security.

This part EN 50126 is applicable

to the specification and demonstration of safety for all software in railway applications and at all levels of such an application, as appropriate, from complete railway systems to major systems and to individual and combined sub-systems within these major systems; in particular:

- to new systems;

- to new systems integrated into existing systems in operation prior to the creation of this European Standard, although it is not generally applicable to other aspects of the existing system;

- for modifications of existing software on systems in operation prior to the creation of this European Standard, although it is not generally applicable to other aspects of the existing system.

- at all relevant phases of the lifecycle of an application

- for use by railway duty holders, railway suppliers, assessors and safety authorities;

• to all safety related software used in railway control and protection systems, including

1. application programming,

2. operating systems,

3. support tools,

4. firmware.

Application programming comprises high level programming, low level programming and special purpose programming (for example: Programmable logic controller ladder logic).

• not relevant for software that has been identified as having no impact on safety, i.e. software of which failures cannot affect any identified safety functions.

This European Standard also addresses the use of pre-existing software and tools. Such software may be used, if the specific requirements in 7.6.4.12 and 8.3.4.1.6 on pre-existing software and for tools in 7.9 are fulfilled.

Existing systems compliant with any version of former EN 50126, EN 50128 or EN 50129 shall not be subject of reconsideration and are considered as compliant with this European Standard.

Railway applications mean Command, Control & Signalling, Rolling Stock and Fixed Installations for Railways (e.g. Electric Power Supply).

Application of EN 50126-5 builds on the results in applying the methods described by EN 50126-2.

NOTE Guidance on the applicability is given in the requirements of this European Standard.