I have, over recent months, been taking an interest in this topic and have been counting sleepers on a large number of photographs in the Highland Railway Society collection (I lead an exciting and enriching life). Forbye the rail lengths mentioned by Graham R in his posting, which gives the lengths of rails in some early constituents of the Highland, I believe that by around the 1890s, the HR standard track was using 30 foot long rails with 11 sleepers to the rail. Most photographs on HR metals show this well into the grouping. This ties in with the drawing at:
http://www.oldpway.info/drawings/1895pw_pl16_HR.pdf
This is a drawing from an 1895 publication and shows rail of varying cross-sections of 76½, 77 and 80lbs/yd. The 76½ lbs/yd section looks awfully like a double headed rail to me; the other two are definitely bullhead.
However, there are a few photos which show 10 sleepers per rail length. There is also one which shows 8 sleepers per rail length in LMS days (at an unidentified location but it shows Mackenzie and Holland signals, so almost certainly ex-HR). I doubt that this would be 30 foot rails. I have also recently found one photo which shows 12 chairs on each rail, but only 11 sleepers per rail length. How was this remarkable feat achieved? By use of joint chairs at each end, that's how. "Jings, crivvens!" I hear you say. You can have a look for yourself at:
https://www.ambaile.org.uk/?service=asset&action=show_zoom_window_popup&language=en&asset=28586&location=grid&asset_list=28586&basket_item_id=undefined
The bottom right hand corner shows a joint chair. Count back from there, if you have nothing better to do.
The use of joint chairs is confirmed by:
http://www.oldpway.info/drawings/1905jt_d42_HR_supported.pdf
This drawing is signed by Mr Roberts in 1905 and shows the use of 90lbs/yd rail. This is quite an increase from 10 years earlier, but it would tie in with the introduction of heavier rolling stock and locomotives at that time and ties in with the general development of British permanent way.
My thoughts are that, once the use of 30 foot long rails was introduced, the Highland used 10 sleepers per rail length for a while, but later used 11 sleepers per rail length. I have not seen any evidence for the use of longer rails on the HR. (This has to be treated with some care, as longer rails would tend to not show their full length in a photo, so the number of sleepers couldn't be counted; I hope that makes sense.) My view is that the use of 30 foot rails continued to the end of the HR. Given the low volumes of traffic on the HR generally, they would remain in use for a long period after the grouping.
On my own layout in 2mm, I have opted for 11 sleepers per rail length on the main running rails, with 10 sleepers per rail length in the yard.
Incidentally, there are a good number of photos of HR trains on the CR between Stanley Junction and Perth. A few of them show 14 sleepers to the rail length. But that was CR track, I presume using the CR standard of 32 foot rails (later 48 feet).
Fascinating stuff! It's remarkable how little attention most modellers give to the permanent way. Without it, there would have been no railway. Locomotives were only replacements for horses, so we could still do without them.
Back to my sad and lonely existence. One sleeper, two sleepers, three sleepers, four sleeper - ooh! look at that lovely bit of ballast...