Mike Hodgson wrote: Given that this method of working places reliance solely on the box in advance, there is no need for a BBI signal to tell the box in rear when it is not in order to offer a train.
I wonder if the above, in essence, is the answer regarding BBI for this time period of c1900?
Unfortunately I don’t have any regulations for 1900 either, but a book I do have entitled ‘The First Principles of Railway Signalling,’ does offer some interesting views on the thinking around the same time as Mike’s regs. above. It was published in 1918 by the Railway Gazette, and written by C B Byles, who was Signal Engineer for the L&Y from 1897 to 1911.
On Blocking Back he writes:It has been explained that, under standard block working arrangements, the line is considered normally blocked and that no movement may take place without the direct permission of the signalman. Experience has shown, however, that there is a risk of signalmen overlooking vehicles standing on the main line and that they may in consequence, notwithstanding the presence of the obstruction, move the indicator from blocked to clear and so permit a train to approach. As a partial safeguard against this possibility, the Blocking Back signal was introduced a few years ago, and the use of this signal is really a partial revision to methods that were in use in the earlier days of block signalling, when the line was regarded as normally clear.
For me, this adds more weight to Mike’s point about ‘there being no need to tell the box in rear when it is in order to offer a train,’ as according to this, the primary purpose of BB, is simply to act as a reminder to a signalman that his own stretch of line contains an obstruction.
Also, given the publishing date of 1918 and the reference to BB being introduced “a few years ago,” does this mean that it is unlikely that BB would have been used in 1900 at all?
With regard to BBI vs BBO, the book continues:The blocking back code is given in a different manner according to whether the obstruction is intended to take place within the protection of the home signal or outside the home signal. In practice, to employ the signal for the former purpose is found very inconvenient at busy places, and it’s use is therefore usually restricted to roadside stations and to certain level crossings. A movement outside the home signal must, however, in all cases be protected by the blocking back signal being sent to the box in rear. The effect of giving this signal from B back to A, would be that A would not attempt to offer a train to B and, if the section were less than half a mile in length, A would himself refuse to accept a train from the rear.
Again, this last paragraph seems to support the idea that BBI is only an aid to a signalman in reminding him of an obstruction, as the lack of it’s use does not compromise his duty to maintain the ¼ mile clearing point. I find the BBO explanation interesting, as the greater importance given to it seems to be driven by the fact that in this scenario, not only is the ¼ mile clearing point completely compromised and unprotected, but that it recognises it’s potential to foul the clearing point of the box in rear also.
Personally, I’m feeling a lot clearer on this now, in terms of how Millers Dale signal box might have been worked, particularly in the way in which shunting moves onto the main line may have been handled. However, where things start to get very confusing for me again, is when considering the operation of the boxes at another station on my layout, namely Marple. My reason for this, is that the station was operated by 2 signal boxes (plus a 3rd one for the yard only) – one at either end of the station – but the distance between the respective starting and home signals for each is only around 300 yards, and thus well under the ¼ mile!!! How on earth would these boxes have been operated??
I’ve added a diagram below, and whilst I’ve attempted to add signals in what I think is a logical manner etc, it is only the track layout that is available to me, so any comments on the plausibility of the signalling would also be valued. There are also a number of trap points which I’ve observed in a couple of photos I have for the period, so I’ve added these also, though their use doesn't seem to be consistent.http://i1260.photobucket.com/albums/ii565/MSheardown/Marple1_zps59ec1f7d.jpg