Back to back locking

[greatnessjason]

Hi Peter
In RGS GK/0202 pg 15, there is a mention of "back to back" locking. What is the history behind such type of locking and what purpose does it serve in terms of the interlocking controls?

[PJW]

Have to be careful re the phrase "back-to-back" locking, because in my experience it can mean one of two slightly different things:

1) Where there are two signals for opposite directions of travel along the same track at effectively the same longitudinal position along the line- almost always suspended on opposite sides of a gantry over a bi-directional line,

2) Where there are two signals for opposite directions of travel along the same track at different longitudinal positions along the line and typically a train length or rather more apart- typically either ends of a through platform line for example.

In case 1) there obviously must be opposing locking to prevent both routes being set simultaneously; however since there is no track circuit between them, then there is only the "Route Normal" or NLR of the one included in the controls of the other; there is no actual opposing route locking since the "Route Normals" are sufficient.

In case 2) the locking does depend upon the specific railway's practice, Typically on NR nowadays there is no restriction on being able to clear both signals simultaneously and if there are two short trains in the same platform there is no reason why they cannot depart simultaneously in opposite directions. However some railways take the view that it could potentially confuse a driver to have a choice of signals and thus not know which way to go. I seem to recall (but stand to be corrected) that Ianrod Eireann take this approach and am pretty sure that traditional London Underground signalling would enforce such locking. I also suspect that many parts of Indian Railway would require such locking. Certainly in the days of pure mechanical interlocking, bidirectional working was minimised and wherever possible when it was needed then differential point setting would be used to lock one direction against the other. Wouldn't be surprised that the rationale for back-to-back locking was as much reflecting such considerations and a means of enforcing the signalman to return as many levers as possible to Normal as it was due to driver confusion.


In the context of the now withdrawn GK/RT0202 (I think because RSSB regard as "single duty holder responsibility" and NR has been unable to republish in a different guise since they are still suffering from the self-imposed "standards freeze") I believe it relates to case 1)

Hi Peter
In RGS GK/0202 pg 15, there is a mention of "back to back" locking. What is the history behind such type of locking and what purpose does it serve in terms of the interlocking controls?