On second thoughts, I was thinking of Sudbury Hill, not the other one. More of an obvious
place than Harrow Road (or Northolt Park) though shops were built around all these Underground stations. But as I recall it, the service was then much the same at all three. Mind you, when I was at school I don't remember ever using this line; if I was going somewhere for an activity or to see someone and not by bike it was almost always by Tube. Out of working hours especially, there were just so few of the others.
This line was always a Cinderella, right from the start. When the
GCR» was built, what demand did it meet? Almost none, in this area - the tidal wave of house-building that was London at the time (and for decades to come) had barely reached Wembley at the time. Wembley's station still had "Sudbury" in its name until 1948, since that was at least a tiny place back in 1860. The stopping service in 1912 was only a couple of trains a day at Sudbury Harrow Road and South Harrow (as it was), both built with bays.
By 1920 the GCR were still only running two stoppers per weekday to South Harrow, though some longer distance trains also called there. The Underground (District Line at the time) was running three trains an hour, but Sudbury Hill and Sudbury Town were request stops, and further out only South Harrow (the real one), Ruislip, and Uxbridge were proper stations: the other four were halts.
The houses turned up en masse in the 30s, so by 1939 the service was much fuller - more trains than hours (though very uneven) and even half that many on Sundays. Northolt Park had been built by then (no loops - it's on an embankment) but it and Sudbury & Harrow Road got just a few fewer trains than Sudbury Hill.
It's obvious that all the railway building in this area was based on the Metroland principle - "build it (the station) and they (the houses) will come". There were good reasons to expect that, but you wonder how much effort went into carefully estimating their number and which lines passenger would prefer. After the war, once most of the market gardens had houses on them, there were only so many passengers to go round. Plus of course the fully-developed electric Underground and buses, as well as the beginnings of car use, which were perhaps harder to foresee in 1905.
In 1955 the service had declined, to 8-10 trains a day and only the odd extra one at Sudbury Hill, and it was similar in 1965. Looking forward to today, I think the "competition" from other railways is still the issue. At Sudbury & Harrow Road not only is there the Piccadilly next door, but half the catchment is as nearer Wembley. So it's back to a limited to-and-from work service. Sudbury Hill Harrow has only the Piccadilly, but why it gets the same commuters' service plus trains all day but nothing on Saturdays is a hard to fathom. The surprise performer is Northolt Park, which now gets a better service than either, weekends included. OK it's not so close to either South Harrow or Northolt, but it's still a big promotion.
All timetable data from Timetable World.