Some questions

[PJW]

I felt the doubts are silly so I asked through the email.Thank you very much for the information given.
Please clarify some more doubts:
1. Whether Lime street control and clearing a call on class are one and the same?
2.Similarly soft calling of points and flank point means the same?
3.Can you explain (What is the alternate solution instead of fail safe design? ) with some other example?

Sugavanam

Certainly not silly questions and exactly the sort of things people should be asking!

In brief (I can add more info perhaps next week)

1. Lime Street control is a specific case for calling-on moves into a terminal platform; actually for various reasons not generally implemented nowadays. "Huddersfield" is different but applies (primarily) to call-on moves into through platforms. Lime Street is not just proving that the permissive section is occupied (as for all call-ons) but to attempt to measure the train to know that the new one will fit in the remaining space. This worked well with a known form of rolling stock of a particular type and when drivers were prepared to go right up to the entrance signal; however there is now more of a mix of tain types and defensive driving means that the front of the train is in a less predictable position so can be more trouble than worth.

2. Soft calling of points can certainly be applied to points on the flank of a route but for which calling and LOCKING as flank is not possible without unduly restricting layout flexibility. Soft calls mean that IF the point can go to the requested position then it is called, but if not then not regarded as too important; point isn't locked so can subsequently be calleed away by some other route. Givces protection if practicable, but not otherwise.

3. This is probably more of a subject for the module 7 topic area, so when I get a chance I'll put there. However compare the TSS to a mechanical trainstop- these need no power to stay in the raised "tripping position"- I think that it is usually compressed air that lowers them when the signal can clear. Almost any credible failure results in the arm being raised (obviously it potentially could jam, seize up, the spring can fail but the whole thing engineered such that these failure mechanisms should be extremely rare). Hence overwhelming probability that the failure mode will be "Up when should be down" = Rightside rather than "down when should be up" (that would be wrongside). Hence pretty well "failsafe"- no doubt much of the residual risk could be mitigated by ensuring that arm does indeed return to be up each and every time train passes signal prior to allowing another train to approach it.
Whereas a TSS loop is proved to be intact and transmitter giving an output frequency before the signal in rear can clear, power could potentially then be lost and the TSS would be rendered ineffective. Also it requires some electronics on board the train to be working etc; conversely in the case of a mechanical trainstop there is very little that could possibly happen to prevent a raised trainstop physically opening a mechanical valve to cause the brakes to be applied. In reality nothing can ever be 100% failsafe, but it is generally true that the more basic and simple a system, the more it is possible to engineer it to be so for all practicable purposes.