2008 Q3 DEGRADED MODE

[Jerry1237]

Could explain what is the meaning of technological fall back system?

likely, we use axle count instead of track circuit failure temporarily. Is it called technological fall back system??

And,

Is POSA technological fall back system???

Thank you.

Yes if track circuits are the main means of train detection but there is addition some axle counter detection (perhaps just one long section covering the area of many track circuits) then that would be a means of operating the railway in a degraded mode by means of a technological fall-back.  The capacity would be much lower than normal, yet trains could stll be operated with the usual high safety integrity.

The POSA aspect is also a technological fall-back system.  Here fewer than the normal signalling controls are imposed so there is less than usual safety integrity (since some of the usual proofs are deliberately omitted), but this is compensated to some extent by the operational rules (e.g. one train movement in the area at one time) and primarily the slow speed of the movement.  It is also far safer than attempting to move trains purely by verbal procedure which would otherwise be the case.

As a reminder to all - the exam is there to allow the candidate to demonstrate to the examiner they understand principles and their application. Cost and practicality are not the main drivers. So, overlaying ACs on a TCd section is a solution. Would it get past commercial? Probably not. It also potentially reduces the system reliability! However, it is an achievable technological solution that has benefits, especially over verbal control. Don't forget to write assumptions and ensure that anything proposed is reasonable. Stick to the point, be brief but make sure the underlying point is very clear.

Jerry