IMO the vast majority were justified
Even after a generation of rising national numbers there are still a number of existing stations/lines that just don't look viable in 2025 - and if they aren't doing well after around three decades of general growth, I very much doubt most lines still closed would be bringing in many passengers
Of the longer distance routes rightly closed, I'd suggest the Dumfries - Stranraer line, the Port Road. Lots of empty countryside. No big cities at either end. No doubt of vital national importance a hundred years ago but the Irish market mainly moved to air decades ago. It's not a realistic diversionary route. There are no large intermediate settlements. Whilst a beautiful and untapped bit of Scotland, there's not the tourist market that helps Highland branches. Not even, to the best of my knowledge, any preservation moment on the line. Being over fifty miles long, it'd be incredibly expensive to reopen
Ignoring the conspiracy theories about Marples/ Beeching and their methodology (which seems primitive in hindsight, but was probably quite appropriate given the record keeping/databases available in the sixties), the suggestions about BR indefinitely retaining the land (which would have reduced their revenue/ increased their ongoing costs through the additional maintenance bills/ no True Believer enthusiast would ever accept selling any of it off even fifty years later) and the reforming ideas (which would have been stymied by the Unions/ only really appropriate on a token number of marginal cases), that leaves us with three categories of line:
1. The ones designed for a specific freight use that had little realistic future once that dwindled (e.g. lines to collieries/ quarrys)
2. The ones close to retained lines, where no large settlement lost it's trains (the GCR being an obvious high profile example, but there were many routes with competition, given the speculative Victorians - did we really need Woodhead AND Hope Valley AND Millers Dale?). Sometimes links were lost (e.g. Rugby to much of the East Midlands) but all of the bigger places kept some train service
3. The ones where towns lost their entire service
The third category is the only one I'd focus on here, if I wanted to build consensus for any Beeching errors
Complaining about the methodology seems pointless (nowadays any Forum member has significantly more passenger data information/ statistics/ flows at their fingertips than Beeching could dream of at the time)
This was in an era when cities like Glasgow/ Newcastle/ Leeds were choosing to turn inner city land into urban motorways, that was the way that the wind was blowing
It was the same overseas too (Beeching's cuts look relatively unremarkable when you see equivalents abroad)
The rationalisation into fewer bigger city centre stations in places like Glasgow/ Manchester seems a positive rather than a negative IMHO
As a thought experiment, if you think BR should have retained and maintained all closed lines for a bit longer 'just in case' then when would you have sanctioned flogging off the track bed of e.g. the Port Road? The line passes through very little on the long schlep from Dumfries to Stranraer, low cost flights have killed of most of the Irish ferry market and even if Ryanair somehow went bust there are no ferries from Stranraer nowadays anyway. Even Stranraer's link to Ayr/ Glasgow only gets enough passengers to fill a single 153 a day.
Would you have retained the Port Road route for ten years after passenger services closed? Twenty five? Fifty? What was the formula that they should have used? Would it have been acceptable for *any* buildings to encroach? For the government to use this government owned land to build a road on sections of? Or would taxpayers have to fund the maintenance of this untouched land forever?
That's my problem with this line of argument. It sounds nice at first glance (Keep the land just in case. No rush. Properly assess things. You never know when it might come in useful). But until what date?
Because otherwise it sounds suspiciously like "we should keep the land forever", just couched in the language of apparent moderation
(argue by all means, but try to be a bit more specific than words like "useful" which appears to be a wooly way of saying "nice to have" - there's always opportunity cost to these things)