Hail again! Ahh...I was given the impression that when a token is passed over a RETB system, the relevant signaller(s) would hold in the release buttons on their appliances causing a "token available" communication to be displayed on the appliances of all trains in the vicinity, with the driver of the relevant train then holding in the "Accept" button on their appliance to take ownership of the token without the need for additional voice communication. 8)
Incidentally...Does RETB use it's own radio frequencies, or does it work using standard communication bands such as those used for CSR?
Once again, no certainty in my knowledge of RETB, but it's my understanding that the signalman issues the token specifically to the intended train; the token then being transferred by driver and signalman simulataneously pressing their relevant button. The request for the token and confirmation of receipt are carried out verbally, as are one or two other bits and pieces ("clear of loop" being one that springs to mind)
Hmmm...Just after I made that previous post, it occured to me that - With the section being pretty short and having no diverging paths or loops along it's length - The signalling system would only need to display a red at both ends (I.E: Only one train permitted in the section at any time in any direction) whenever the block is occupied...Meaning that the train itself would act as a whopping great "token" in it's own right!
Of course, this assumes that the section is actually controlled using the "one train only" principle. I suppose (R)ETB or a similar system would be provided if the section needed to be capable of accommodating multiple trains travelling in the same direction at the same time - But given the block length, this probabally isn't necessary.
If there are intermediate signals, then there's no reason why more than one train can't be on the single line at once. There'll be additional controls to make sure that everything's travelling in the same direction (i.e. once one train is allowed into the section in the Up direction, the protecting signal can't be cleared for a Down train), and the function of ensuring that the trains are kept a safe distance apart will be carried out by the intermediate signals. See Waterloo Simsig for this in action - set a route along the Windsor Reversible and note how all the automatic signals come 'off' all at once, and how you will be prevented from setting a route onto the line in the opposite direction.
It's also possible with ETB - Berney Arms (between Reedham and Yarmouth) had a box to break the section. When it was open, there was a token section either side, but no passing loop. Whether there were controls to prevent the signalman to accept a train from both the Yarmouth end and the Reedham end at the same time, or whether it was left to the signalman's common sense, I don't know!
I suppose the same thing would be possible with RETB, but I don't know whether it's actually been implemented anywhere. There's one or two isolated stop boards that I can think of - I believe there's one approaching Crianlarich from both Oban and Fort William, for example - but these are to permit trains to approach Crianlarich from the loop in rear whilst the junction is still blocked by a conflicting move. In that case, the authority would be to proceed only as far as the intermediate stop board, where a further token would be required. A 'long section' token would be issued at the loop in rear if the authority to proceed was right into Crianlarich station, in which case there'd be no need to stop at the intermediate stop board.
Hmmm...Well, my idea on this point was aimed entirely towards the RETB system (Could ye imagine trying to obtain/hand-over a physical token at 125mph?
) but the following is kind of what was going through my mind when I asked that question. For this, assume that ye have a Voyager approaching a RETB controlled section at 125mph, and that it will maintain that same speed throughout unless instructed otherwise:
I think I can say with almost complete certainty that any such line would either be doubled or track-circuited throughout! RETB was introduced as a method of signalling to reduce costs on a few marginal lines, so is only really suited to those lines with a sparse service and low speeds.
...Although one possible flaw that has just come to my mind with regards the RETB system - Though it would have to be committed by human error - Is that as no physical tokens are used, a token could (In theory) be released by a driver before he has actually exited a section if he wanted to try and speed things up. Obviously if such a thing were to happen (Perish the thought!) the token could be handed to another train that is about to travel down the same line in the opposite direction, with potentially unfortunate consequences occurring when both trains encountered each other!
hock:
I suppose that's entirely possible, and it's these little flaws that make it unsuitable for anything other than a line with a few trains a day, pootling up and down at a nice leisurely pace! Given that all communication is 'open' (so can be heard by drivers of other trains), I'd expect the driver of a train waiting at loop A would be most surprised to hear the driver of another train claiming to have just arrived there and cleared the single line!
Two additional RETB questions that I now have though, if I may: 8)
- Looking at one of the linked PDFs (Concerning a near miss in the vicinity of Aylesbury) it becomes apparent that physical tokens are also used as keys to unlock ground frames and other manually controlled loop arrangements, effectively handing possession of the token to that loop and any train upon it.
Now as a traditional ETB system makes use of physical tokens that can double-up as frame keys, this is a simple way of safely "locking out" an entire single-line section to allow for locomotive movements to take place. Is the same principle used in RETB systems (Perhaps with a token-holding appliance that will only unlock the ground frame once it has obtained the token), or are these supplimented with a physical frame key?
- What would happen in the event of a token appliance experiencing a critical failure of some description whilst it was in possession of the token, meaning that the token for that section of line would effectively be lost? hock:
The arrangement at Aylesbury (and Glaisdale/Grosmont/Whitby a couple of posts ago) does indeed allow a token to be released for shunting on the single line at an intermediate point. It also allows trains to be 'shut inside' for other traffic to pass, perhaps while they shunt, and then to obtain another token to continue their journey on the single line. I don't see why that would be impossible with RETB...I just don't know if it's done! It's certainly possible to obtain shunt and engineers tokens (to shunt at a passing loop and to take possession of an single line section for engineering purposes) - if the worst comes to the worst, and it's possible for the system to 'lose' that token, then the signalman should still have a record of its issue?
Phew! What a post
I've just found
this, which may be of some interest. I knew there was an article somewhere, but I couldn't find it. Wish I'd found it before I typed all that out
Tom