But as previously mentioned, where do the buses come from? How do the staff get to the buses? How do the trains start running again when crew are all away driving buses?
Nobody seems to have an answer for this
It's one thing to arrange coaches for a planned disruption (e.g. planned engineering works will close the Dibley branch line, so the TOC has weeks/ months of advanced warning to arrange a certain number of coaches for a guaranteed period of contract - that's something that can be arranged) - my local bus route used to be run by some of the large fleet of buses that were initially procured for the "rail replacement" work needed to cover the WCML upgrade through Stockport around twenty years ago - some of these contracts that last for months are rather lucrative and stable work for bus/coach companies who can plan ahead...
...but the kind of urgent/ short-term disruption talked about in this thread (wires coming down in central Glasgow, meaning no trains can run - this is a route with a "metro" frequency and the wires coming down is obviously something that happens pretty quickly without the luxury of time to plan ahead)
So, what do we do? Have a fleet of dozens of coaches sitting within spitting distance of central Glasgow stations, all day/ all year? Plus the drivers who can drop everything and go off to take these vehicles anywhere that trains go (which, in some rural areas may mean roads not particularly suitable for big coaches - e.g. in the example of Glasgow, how easy would a driver find it to serve all stations on the West Highland Line in the even of disruption?)
And I mean dozens of coaches - a double decker bus has a broadly similar capacity to a single railway carriage - so in the event of a line with six coach trains every few minutes like the Queen Street Low Level route that the OP was talking about - you're going to need lots of drivers/ coaches. And, given that coaches serving each station are going to take longer to get through congestion than a train could cover the distance, we'd probably need even more coaches just to provide an equivalent capacity.
Modern coaches too - it was acceptable in the Fraser Eagle days to rustle up any old vehicle but the "accessibility" requirements for buses/ coaches means that there's a much smaller pool of suitable vehicles that can be used on this kind of work - and modern coach companies don't have as much "slack"
All those coaches, all just sitting around all day though, with coach drivers too, just in case the wires suddenly go down, or there's a body on the tracks or some other unforeseen incident?
And, if you want these run by rail staff then do you expect the same need to maintain route knowledge etc? Will the rail Unions insist that staff are all sent on refresher courses to cover every permutation of driving between stations - in which case that means taking a number of drivers away from trains to keep their boxes ticked.
I think that this is an important thread because it shows the difference between the people who accept the railway is restricted by practicalities and those who think that everything should revolve around the needs of the railway (even if that means dragging vehicles away from school bus services, because The Railway Is King) - the kind of people who happily ignore billions of pounds in subsidy because they see the railway as untouchable.
Of course nobody wants to be left in the lurch, I'd be annoyed if a TOC was unable to rustle up a replacement vehicle at short notice, but I'm realistic enough to realise that the cost of ensuring dozens of spare coaches at each major train station would bankrupt things - it's a bit like the glib statement that "passengers prefer direct trains" - of course they do, but it's not practical to serve every individual market with one.
Obviously TOCs should take reasonable measures, no argument, if help can be sourced then it should be, but it's not always going to be easy - and in cases where the disruption may only last a couple of hours then bus/coach companies aren't going to be cheap