A few questions around cutting Girvan-Stranraer and replacing it with an "express coach":
- Assuming the GLC-Ayr services extend to Girvan, what'd serve Troon-Kilmarnock?
- What guarantees are there of the bus service being retained long term? It wouldn't have the level of permanence and political support that a railway does.
- What frequencies would you run such a bus service at? Who'd run it - a coach operator on a contract, or a local bus operator? Quality and long-term viability would heavily vary based on what you pick.
- How do you ensure both reliable, predictable services to Stranraer and good ferry connections at Cairnryan (to Stena, but also possibly P&O)? If you can't do both, how'd you prevent excessive duplication of services?
- How would you guarantee rail-bus connections at Girvan or Ayr? Reliability is one risk (particularly with A77 traffic), but also I doubt Girvan's small bus-only loop with a little shelter would fit a coach, and even if it did, it wouldn't be a particularly comfortable/convenient transfer compared to a direct train.
- How would you serve Barrhill? I'd suggest a Girvan-Newton Stewart coach service (departing alongside a Stranraer coach) but this is another route where managing capacity, frequency and long-term viability would be difficult, and subsidising a tendered coach route would be a risk regarding quality and reliability.
All these would have to be addressed properly for bus replacement to work. There's no cheap option other than abandoning Stranraer altogether and waiting for it to depopulate, a good bus on an obscure route still requires subsidy. Ignoring any of these issues runs the risk of replacing a decent train service with a crappy bus service, which is something we should leave in the last century.
Its a pity nobody put these questions to Beeching and all the other bright sparks who over the years think closing railway's or not re - opening them and putting on buses is the answer

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We then might still have 20000 miles of network to use

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Here's a couple of ideas that could be looked at:
1 - How much would it cost to put in a triangle junction East of Stranraer and then a single track to Cairnryan so trains could run Glasgow - Stranraer then reverse to Cairnryan and vice versa?
Because I know you would not get thousands of passengers but if there was some working together between Scotrail, the Irish railway's, the ferry company's, various Government/local authorities both in Scotland and Northern Ireland/Ireland and tourism body's on a targeted advertising campaign could there be some scope in offering foot passenger's some sort of daytrip/holiday package which would be: Tickets from Scottish/Northern England stations - Stranraer, ferry, then train from Larne - Belfast/rest of Northern Ireland/Ireland?
2 - When the cars and lorries get off the ferry obviously the are not all going to exactly the same place and that to offer a shuttle type train to take every car or lorry to where they want to go in the UK that was on the network is not practical but could there be enough of a demand and the expense justified for raising bridges etc to offer a piggy back/motorail service between Cairnryan - Glasgow/Kilmarnock for the M74?
3 - If Stranraer station is relocated to the centre of Stranraer then you are left with a decent bit of track, a station building and a large bit of land to store stuff.
If the the SVR trial is a success and other railway's follow suit the logical next step would be for a heritage railway to come back on to the network full time, or as is regularly discussed a lot heritage of railways are looking "creaky" and potentially it is only a matter of time before one fails.
Either way there is potentially a full inventory of locomotives coaches, wagons, signalling equipment, buildings etc up for grabs, also most councils seem to be very supportive of heritage railways as a way to bring in tourists and while I don't know where the nearest heritage railway to Stranraer is I'm guessing its a very long drive there and back or doable by public transport but only as a overnight visit?
So, what could happen is the council encourages a heritage railway group to start up, the council, the group or together they buy the assets of a defunct railway and move them to the old Stranraer port site and set up a Heritage Transport Museum, get hold of some old cars, buses and trucks. Maybe get some local classic vehicles owners clubs to set up there as their base and perhaps offer to let people have a go at driving them, see here far an example:
www.drivedadscar.com
Monday to Friday it would be a Didcot type of set up where the heritage trains run between the old Stranraer station and the new one and on weekends the trains would run all the way to Ayr or maybe even Glasgow (think NYMR/Whitby).
At the moment I cant think of any reason why I would want to visit Stranraer but offer me a ride on a heritage train from Glasgow/Ayr to Stranraer, a look around a transport museum/drive the exhibits and then a ride home again I would be first in the queue.