It was a relic of the LNWR and later the LMS which ran quite a few services, including portions of London trains, from Stockport to places like Oldham and Huddersfield.Its precise purpose, as the original BR service was, would be to make connections between South Manchester services and North Manchester ones including TPE. Though I do think running it to Vic would be more useful than Staly.
The previous service kind of made sense, as a box-ticking exercise, since it took a turn that was running ECS from Stockport back to Newton Heath (via Denton) Monday to Thursday, so little extra resource required to divert it via Stalybridge on a Friday.
What does the new (return service) run off though?
The full ECS working on Mondays to Thursdays was 5J05, 0833 Manchester Piccadilly to Newton Heath TMD via Styal, Wilmslow Reversing Siding, and Denton. On Fridays it became 5J45, 0833 Manchester Piccadilly to Stockport via Styal and Wilmslow Reversing Siding. So instead of it continuing via Denton to Newton Heath, it was indeed a relatively minor diversion via Stalybridge on Fridays.
http://www.charlwoodhouse.co.uk/rail/liverail/train/10809396/16/05/18
But that begs the question as to why 5J05 existed in the first place. It could hardly have been for retention of route knowledge Stockport to Newton Heath, since there are other workings between those points anyway. Could it have been simply to keep a path open for the Friday service? That suddenly makes it a very expensive once per week service.
I can't find any equivalent to 5J05 on the current workings.
Will the official easement of allowing the return journey via Manchester be retained, in view of there now being a return trip ? Or is this an exercise in trying to get the said easement abolished ?
Is Stalybridge still known as Stalyvegas?