Safety assessment of rail systems

The signaling system is a system to ensure train operation safety. Therefore, it
must be self-safety in its whole life cycle. Fail-safe concept was put forward for
the railway signaling system in the early 1900s. Safety assessment for the
railway signaling system begins with the start of the system design. From the
core system design to redesign of an application signaling system, from the
prototype of the core systems to the minimal system, from manufacturing to
installation, from operation to maintenance, safety assessment must be taken
during the whole life cycle. This is the main reason why a signaling system is
complicated and high cost.
There are always two groups of persons in this area. One is to implement the
signaling system. Safety assessment is done by another group to ensure the
system‘s implementing to be monitored. Moreover, the second group should
involve from the beginning of the system design. In other word, the whole
process of the signaling system design, manufacturing, installation and operation
must be monitored and assessed. Methods and principles for software
engineering must be used for the files management and flow management to do
safety assessments of the signaling system. For a big project of railway
signaling, the third professional company is invited to do the safety assessment
for the project.
What is the meaning for RAMS? The RAMS means Reliability, Availability,
Maintainability and Safety of the system. According to EN50126 (CENELEC
1999), the definition of RAMS can be found easily in Theeg and Vlasenko [6].
System reliability is defined as the probability that the system can perform a
required function under given conditions for a given time interval. System
availability is defined as the ability of a system to be in a state to perform the
required function under given conditions over a given time interval, assuming
that the required external sources of help is provided. System maintainability is
defined as the probability that a given active maintenance action, for a system
under given conditions of use, can be carried out within a stated time interval
when the maintenance is performed under stated conditions and using stated
procedures and resources. System safety is defined as fail-safe requirements that
system cannot give dangerous output when a given fault occurs. Reliability and

maintainability are both probability values which lead to failure and maintenance
rate respectively, related to a defined time period. The signaling system is
required to be with high availability, i.e. low failure rate and high maintenance
rate. System safety is the system quality requirement, and different with
reliability. When the concept, as reliability and safety of railway signaling
system, is discussed, there are still some of different views as in Ning et al. [2].
In order to ensure the requirements of RAMS for the signaling system to be
satisfied, fault-tolerant design, fault-diagnosis and fault test are applied in the
design and redesigned for signaling system development. Comparer is often used
in the design of signaling system to fulfill the fail-safe requirement of the
system. The comparer can be implemented both by hardware and software.
In the safety assessment, a simulation system can also be used to testify if
RAMS requirement of the signaling system is performed. It can be used for
safety assessment of the core system, the minimal system and the application
system of signaling system. Fault set and fault models of signaling system are
analyzed and built.