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You can get Advance fares at
East Coast. These commit you to the booked train(s). Prices (to Greenock Central) vary depending on time of travel but currently available tickets, as of time of writing, start at:
£20.20 from Durham
£19.60 from Sunderland or Seaham (same price from Seaham but fewer options)
£18.60 from Newcastle
Sunderland/Newcastle cheapest tickets are available early afternoon; Durham tickets are available in the mid-morning.
Trains Can Be Cheaper suggests a cheapest flexible fare splitting at Dumfries from Seaham, Gretna Green from Sunderland and at Carlisle and Gretna Green from Newcastle or Durham. Tickets for these are £35-£39 depending on where you start from. That will be quite a bit slower than on the ECML, and involves travelling to Carlisle along the Hadrian's Wall line, then taking
the Glasgow South-Western Line to Glasgow Central, changing there for Greenock. You could use these tickets on the 13:23 and 17:16 Newcastle-Glasgow trains, which take four hours to get to Glasgow.
