CBTC+Fall-back

[PJW]

Thanks for reply. By assuming that system achieves a sufficient level of safety, based on the CBTC concept, ıs there any quantitive method to evaluate "availability" ?Point of view each component failure and impact of failure on the train operation ,as a consequence loss of revenue, comparing inital and operational costs of fall back system...How method should be used to decide fallback requirement?

Yes that is the approach which should be taken. 
Of course the reliability performance of each component can only be an estimate based on theory predictions and trying to compare with already commissioned systems having some similarity, but whereas this might be quite good for random ones, by their nature systematic errors giving reliability problems may not become evident until the right (or wrong!) combination of factors arise.
Certainly the overall system availability can be derived from combining the various component system reliabilities, taking into account the architecture giving diversity and being alert to any common mode failures.

Then other modelling must be used to estimate the impact, how long it will be to resume services etc. so more now in the realm of operations than engineering per se.  Probably not easy to get an estimate that all will agree on but yes in principle then one has a figure to see whether it is worth investing money re reduce the impact, either in a fallback system.to reduce the consequence of the main system failure or in adding more redundancy into the core system to improve its availability to reduce likelihood of failure.