... the terms 12" timbers etc are starting to make sense, in that I can see the differing timbers and relate to them
In general terms, pre-group railways used 10" wide "wood strips" for
sleepers in plain line track and 12" wide "wood strips" for
timbers in turnouts / diamonds / compounds / tandems and other S&C work. There were exceptions and Hartley Hill is showing two examples at this time although not necesarily in the photos to date.
From circa 1900 (exact date not known to me) the LNWR began to use 12"
sleepers adjacent to a rail joint in plain line track where the rail was 60'0" in length. The reason for the wider sleeper is to accomodate a "joint" chair with a wider base. What I have not been able to determine is LNWR practice where a 30' rail length is used post circa 1905.... I would expect that the 12" wide
sleeper would be used adjacent to the joints between 30' rails, just have not found any evidence as yet. Similarly, one might surmise that the early 60' track panels might have been laid with 10" wide
sleepers throughout prior to the introduction of the 12" wide joint
sleeper.
Pre-1900 the LNWR used 14" wide
timbers in switch and crossing work, moving to a 12" wide
timber at some later date and before 1909 (PW drawings of that date appear to show 12" rather than 14" timbers). Where things get interesting - and this has yet to be incorporated into the Hartley Hill plan - is that the LNWR used interleaving of
timbers in turnouts. In such examples the switch and crossing
timbers were 14" wide... and the
timbers under the closure rails were 10" wide (
timbers not
sleepers!!!!!).
And just to add some variety to the spice.... GWR S&C practice was 12" timbering throughout turnouts except for 14" timbers under the nose of the Vee and under the toe of the switch.
BTW - free paracetemol with every PW S&C lesson

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regards, Graham