I would say Stockport to stalybridge weekly parliamentary service would take some beating. Think it has been that way for at least 20 years. I'm sure someone would be able to say when it last had a better than weekly service.
I cannot remember the date, but it was when the Trans-Pennine service from Leeds and Huddersfield was diverted to Manchester Piccadilly, rather than running through Miles Platting to Manchester Exchange, and later, Victoria.
Before then, the Stalybridge-Stockport service was almost hourly, to save folks having to walk between Manchester Exchange/Victoria and London Rd/Piccadilly.
Indeed, there were even through services via Huddersfield from Leeds City, Bradford Exchange and Halifax to Stockport, to save the trek across Manchester.
I remember once catching the evening return from Stockport, about 7.30pm, to Halifax via Huddersfield. It even used the high-level line avoiding Guide Bridge. I remember looking-down on the junctions at Guide Bridge as we passed over.
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The Newquay line had an increase in service approx 5 years ago, including an early train (previously absent) for a year until DFT decided the Unit should be used elsewhere, Also, at about that time, an all year round Sunday service was introduced.
Blaenau Ffestiniog has been pretty static over the years.
When first turned-over to Derby Lightweight dmus, two sets were in use on the line, passing at Llanrwst or Betws-y-coed.
The service was simplified with the introduction of ATW's running the branch on a "one-engine in steam" basis.