I was using Carstairs in the 1970s, changing for Edinburgh, and the current layout, completed for the 1974 electrification, is pretty much what was put in for that, unless someone will correct me.
Always seemed a bit strange. The Down main which passes away from the platforms on the south side was realigned for high speed, but althouth there is a second track there used as a down loop, the Up main continued to curve around the island platform, at medium speed, which was heavily banked for the curve. Up trains at speed used to come as a surprise to connecting passengers waiting on the narrow platform, and drivers would generally want to give a good horn on approach if they saw a waiting crowd.
The Edinburgh line joined the Up main by an extremely tight curve which had what, in road terms, would be called "adverse camber" by the points to allow the Up main itself to be superelevated for the curve. Then there was an equally sharp crossover to the Down platform, isolated on a loop. Always seemed strange that the Edinburgh line into the station was reduced to single track, while the link directly southwards, which I never saw used, retained double track.
Until 1974 there was a very sparse service on the Carstairs-Edinburgh line, but afterwards there was a good all day operation of trains, to both Liverpool/Manchester and to Birmingham, and they very rapidly became well used. The layout suited trains dividing between Glasgow and Edinburgh, which of course nowadays has been completely given up, all the Edinburgh trains take the direct line south, and there's just an odd residual service to Carstairs station itself. As described above there was originally an even more direct and straight link from the Edinburgh line to the south, whose earthworks can still be seen, rather strangely this was given up as long ago as the 1860s when the branch from Carstairs eastwards to Dolphinton (a real no-hoper, seemingly just built by the Caledonian to spite the North British coming to Dolphinton from the other direction) was built across it at right-angles.
Ideally the main Glasgow lines should both pass the station on the south side, the Edinburgh line should reinstate the 1860 link, and the station either be put on a loop or be considered if it is really justified any more. Going by the odd few trains nowadays, probably not.