Surely electrification from Manchester to Stalybridge is seen as via Guide Bridge which only leaves the section from Guide Bridge to Stalybridge to electrify.
The line from Manchester Victoria to Stalybridge via Miles Platting is not what one could view as a well laid out line from Miles Platting junction onwards with good foundations with items such as Ashton Moss to cross. What speed limits do currently apply in certain sections and how is it proposed to upgrade to improve on that position. What would eventually happen to the existing road crossing of the line in the Clayton Bridge area?
As I posted previously, via Miles Platting is a fairly slow route. However it will become the preferred route for Transpennine services under the Northern Hub scheme. The Guide Bridge route isn't particularly fast either and has more intermediate stations - these could be bypassed by reinstating the third and fourth tracks, but there would still be the problem of having to cross all tracks at Piccadilly to continue towards Liverpool or the Airport.
The acceleration of Transpennine services through Manchester under the Hub is mostly down to less padding for recovery from delays* and only one station stop, rather than any actual faster running.
*though, as also posted previously, I suspect some of the padding will need to go back in to make the multiple flat junctions west of Victoria workable.
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(Although originally Oldham was on a more important through route.)
Oldham has never been on a through route of any importance.
The Ashton and Greenfield routes joined end-on between Glodwick Road and Clegg Street but most trains on both routes terminated at one of these stations and the few that ran through were occasional locals or specials that couldn't be accommodated via Stalybridge. The Greenfield route could have been linked to either the Ashton or the Hollinwood route to provide an alternative Transpennine service, giving Oldham trains towards Leeds and non-stop ones to Manchester. But instead it was simply seen as a continuation of the Delph branch and both were closed due to the intermediate communities being served much better by bus.
The Hollinwood and Werneth Incline routes mostly ran through to Middleton or Rochdale but this wasn't particularly useful as a through route as Manchester to Rochdale was always quicker via Castleton. It wasn't even much good for diversions, as trains from Manchester via Oldham would arrive at Rochdale facing back towards Manchester.