(05-03-2010, 06:48 PM)interesting_signal Wrote: I'm having a bit of trouble identifying some opposing routes.
Looking at the diagram on page 44 of "Control Tables PJW.doc" (section 7.3.4), if a train is travelling from signal 22 to 18 and only occupying SF track having cleared SG, 931 points will be free to move reverse, and so the route from 24 to 18 could be set (although it wouldn't get the aspect because of the train on SF). In the route control tables for 22 signal, would 24 be an opposing route?
Any comments or hints would be appreciated.
Just to confirm that which Peter has told you; they are not opposing routes. There is also no need (in the vast majority of - but not all- interlocking technologies) for the routes to lock each other.
On Network Rail (but not all railways) it is perfectly acceptable for a signaller to set a route prior to the aspect being able to clear- it is really just then like an auto signal which will eventually clear when the train detection sections permit. However it is not permissible to set a route at a time when
a) points it requires are locked in the other position (because it would inevitably result in points moving apparently "by themselves" sometime later that may be inconvenient / dangerous if it conflicts with the signallers intentions at that later time)
b) when there is an opposing direction move taking place (since this would be potentially setting up for a head-on situation. Firstly the risk given any wrongside failure of train detection is much greater; secondly two trains - particularly if pulled by engines rather than being multiple unit stock- facing in opposite directions would at least be an operating inconvenience).
As Peter said, by the time there is a train on SF going to the left- it matters not whether the first came from 22 or 24- it is simply a following train.
Some other railways do require all the train detection to be clear before permitting the route to set. Indeed this is one of the controls within Network Rail's Automatic Route Setting. A computer that is constantly monitoring traffic wastes little time in setting a route at the first opportunity and therefore there is no advantage in permitting it to do so "early". However a signaller has many other things to do (such as answer phone calls) and unless they are permitted to set routes prior to the signal actually being able to clear, the chances are that trains would be delayed. For very small signalbox area with few simultaneous train movements this would be an insignificant consideration; where one signaller is responsible for a large area and the signaller has much on their mind and many possible distractions , it would be another matter altogether.
It is only fair to state that there are however a few situations when tracks ARE proved clear (or indeed occupied) before permitting a route to set; for example where there are M/C moves into a platform.
Similarly there are a few very specific situations in which there is "same direction route locking". Don't worry about these for now- you need to get secure on the basic locking to achieve a Pass. Later, if you are hoping for a Credit or Distinction, then it would be worth understanding pre-set shunts and GK/RT0044 "Huddersfield" locking.
Signalling does tend to attempt to solve certain particular problems and as a result get complicated- when someone is initially learning it is very very easy to get them confused by the more exotic. In fact only last week I met for the very first time some locking that has been invented to solve a particular site issue; once I appreciated what it was actually trying to do, I concluded that it was certainly appropriate both there and potentially some other sites- however I resist the temptation to explain "Goodington Locking" which is a term that almost no-one will have heard of! Signal engineers, like other enthusiastic specialists, often take excessive delight in the "oddities" that are the "icing on the cake"- however it is far more important to put effort into producing a decent cake in the first place.
