This is an excellent example of one of the many problems introduced by the RDG's insatiable craze for eliminating "Any Permitted", and replacing this with either route dot (".", displayed as "Any - Permitted" on some systems) or "via X". Not to mention some of the crazy repercussions this has with the Routeing Guide and booking engines.
The through fares from Buxton to Marple are all routed "via Manchester". Sensible, in one sense, because that is how they are priced. But totally nonsensical as soon as you look at the fastest journey opportunities between the two stations.
A website like
www.fastjp.com, which purely looks for the fastest possible options and isn't constrained by ticket routes, the Routeing Guide, easements and so on, will clearly show that the fastest route between Buxton and Marple is, almost invariably, changing at New Mills Newtown and New Mills Central, with a walk between the two. The journey time is approximately 1'05" this way.
It also shows itineraries, that are almost entirely overtaken by the above route, where you change at Manchester Piccadilly. The journey time is approximately 1'45" this way - so, unless there is a specific reason why you can't, or would prefer not to, walk between Newtown and Central, I would have thought that travelling via Manchester is not an attractive option.
The issue is the route of "via Manchester" though. For Routeing Guide purposes, this means that the ticket is effectively two tickets in one: a Buxton to Manchester ticket, and a Manchester to Marple ticket. You have the right to stop short, or start late, on either portion, as none of the tickets in question have any break of journey restrictions.
One valid route (as per the Routeing Guide) for a plain Buxton to Manchester ticket, is via New Mills Newtown and Central. That is because it is
shorter than the shortest route that involves
solely using trains (via Stockport), at 24.31 vs 25.66 miles. But that, too, is affected, by a ludicrous ticket route - "via Stockport". It is only through the saving grace of a ticket route easement, which permits tickets routed "via Stockport" to be used via Romiley too, that this route is actually permitted.
So, to answer your question itself, yes, this is a valid route - as you can simply stop short at Marple, before ever reaching the via point of Manchester.
But the story goes further. If you enter 'via New Mills Central' into most booking engines - such as Northern's website, LNER's website, TrainsCanBeCheaper (the non-splitting variant of TrainSplit), or indeed something like The Trainline - it will simply tell you that there are no valid routes.
This is, as explained above, preposterously incorrect. It appears that most booking engines simply don't take into account the fact that it is permissible to stop short on the ticket in this way. To give them credit where it's due, this is no doubt a difficult thing to implement.
But then other websites have other results. If you use Chiltern's website, for instance, it will sell you tickets for the itinerary that FastJP produces - but in fact it is not one ticket that they are selling you, but instead a combination of tickets (which happens to be cheaper than the through fare, if you are avoiding Manchester).
And if you use an ATOS MixingDeck based site, such as TransPennine Express' old-style site, you can be sold the 'via Manchester' fare with an itinerary via New Mills Newtown, Central, into Piccadilly, and back out to Marple - i.e.
doubling back between Marple and Piccadilly (which is perfectly valid, given the ticket route)! Admittedly, getting this kind of itinerary to show up is a little complicated, as you first have to enter the journey via New Mills Central in the "timetables only" mode, where it will initially appear to have ignored the via point, and then you also have to add "calling at Manchester Piccadilly", and it will then produce the itinerary, which you can then select to buy a ticket for.
This has to be one of the craziest examples of the problems that the dysfunctional rail industry's ticketing systems, and in particular the cack-handed "attempts" to fix it, can cause.
