The service, which came back from closure 18 years ago, is to be increased to up to double its current frequency, The Scotsman has learned.
Three additional trains a day are to run to the Royal Mail’s depot at Shieldmuir, near Motherwell, when its giant new parcels centre opens next year at Daventry in Northamptonshire.
The news came in a presentation by Network Rail at the Rail Freight Group’s (RFG) Scottish conference in Edinburgh this week.
The extra services are expected to come on top of Royal Mail’s three trains a day between its depot at Willesden in north west London, Warrington and Shieldmuir which carry some 300,000 items a day.
They are operated by Royal Mail's dedicated fleet of 15 windowless, sealed trains which were specially designed to carry mail containers.
The four-carriage trains, which operate on the west coast main line, have already been upgraded by reducing the number of doors to increase their capacity by nearly one third.
In 2003 it looked like the end of the line for mail trains when Royal Mail cancelled all its contracts, including the “travelling post offices” immortalised by the 1936 documentary about the London-Glasgow service which featured the WH Auden poem of the same name and music by Benjamin Britten.
Postal workers on the specially-adapted trains, which started running in 1838, sorted mail into pigeon holes en route – in addition to collecting and dispatching mail bags suspended from trackside poles, which continued until 1971.
The last travelling post office ran in January 2004, but mail trains with no staff on board were reinstated for Christmas 2004 and then continued.
For a time, a service was also added on the east coast main line between London and a Royal Mail depot near Newcastle.
The Royal Mail's latest expansion of the service is understood to be part of its planned transition to become international business focusing on parcels.
The company has benefited from an online mail order boom fuelled by lockdown restrictions during the Covid pandemic.
The Royal Mail new 78,000 square metre hub – the company’s biggest parcel centre – will be part of the third phase of the Daventry International Rail Freight Terminal.
Its automated sorting system will be capable of handling more than 1 million parcels a day.
Welcoming the expected new cross-Border trains, RFG director general Maggie Simpson said: “Moving post, parcels and packages by rail is a huge opportunity, making online retail cleaner and greener and keeping lorries off the roads.
"We welcome the investment in new facilities to support this, and look forward to seeing new services starting in the coming months.”
A Royal Mail spokesperson said: “We have no comment to make at present.”
Mail has been carried by rail for nearly 200 years, with services starting between Liverpool and Manchester in 1830.
Other firms, such as Orion, are seeking to develop rail freight by converting passenger trains to run on routes such as Glasgow-London.