Very sadly, there seems to be little information readily available about the PW practices of Scottish railway companies, in particular as to timbering of their P&C work. I’m always pleased, therefore, to see how others have gone about representing this, particularly when it come to complex formations.
Mark, I see you’ve managed to use only sleepers all the way through the tandem, and that leads me to wonder how you determined the sleeper positions, as I see some potential problems with the fitting of chairs to this formation were it to be reproduced full size.
I can only speak as to what I’ve learned about North British practice, but I suspect the Highland may have adopted a substantially similar approach to construction of its common crossings as the NBR. Since the block chairs castings incorporated point and splice rail displacements and angles appropriate for particular crossing values, they effectively determined the pitch between each chair, and hence the pitch between sleepers. From prototype observation I deduce that each of the block chairs on a crossing were mounted at a standard pitch of 18” (+/- 1”, according to my measurements).
You can see this constant pitch between block chairs in the attached picture of part of a drawing of NB P&C work obtainable from http://www.oldpway.info/opw_drawings.html#HR_dwgs
I’ve marked this to show how the angular displacement of the sleepers relative to each other is also applied to the block chairs, suggesting that there is a difference between the set of block chairs for a left hand crossing and that for a right hand. Of course this wouldn’t be necessary if the sleepers/block chairs were laid out parallel to each other through the crossing area, but on the North British at least it appears that all sleepers were laid as nearly as possible at right angles to the stock rail they were supporting.
Here’s the problem I see in chairing your tandem:
I’m not sure that some of the block chair positions I have overlaid on your photograph can be adequately mounted on the timbers as shown, but in any case there are some significant variations in the pitch between block chairs in both of the crossings shown, which I suspect could not have been accommodated by a standard set of castings.
To develop a design for a 4mm tandem based on North British practice, I took the 18” pitch between block chairs as my starting point for the sleeper positions. It very soon became apparent that this presented major problems: it simply was not possible to position sleepers at the required pitch at all of the crossings if each sleeper was laid at right angles to the stock rail. I was, therefore, relieved to find a picture of a tandem at Helensburgh which indicated that the North British would, in such case, include some crossing timbers in the formation, and seized on this to justify a mix of interlaced sleepers and through timbers in my design, as shown in the attached picture:
Hope this may be of some interest, as interlaced timbering of a tandem can be quite a problem.