Although the 23 remaining Class 144s are still in operational service, they have become surplus to requirements as Northern are running a skeleton timetable during the lockdown period.
Northern are paying a storage fee to the Keighley and Worth Valley Railway, who will keep the units on their preserved track near Keighley Station until they can return to mainline service.
The buffer-to-buffer collection assembled at Keighley represents all of the Class 144 Pacers ever manufactured, as the majority of the Pacers leased by Northern are Class 142s. They have been running since 1987.
The agreement was reached as there is no space on the main rail network to store redundant stock while so many trains are not in use.
The diesel units are due to be retired in August when the phased withdrawal of the Pacers ends.
Some railway experts speculated that the stabled Pacers may not ever return to service if lockdown restrictions are extended past their retirement date.
The Keighley and Worth Valley Railway have had to suspend services and postpone their summer season, and have expressed concern that the volunteer-run heritage line could cease operations in 2021 unless they receive government help.
The KWVR had already incurred a large repair bill after their engine shed in Haworth was flooded during Storm Ciara in February, damaging several diesel locomotives.
The cancellation of the Haworth 1940s Weekend, which brings large numbers of passengers travelling to the festival by steam train, is also a major financial blow.
The units' arrival is a fitting swansong, as they were originally produced specifically to work on local lines in West Yorkshire. They called regularly at Keighley Station until the Airedale Line was electrified in the 1990s.
Although the Pacer fleet was due to be withdrawn by the end of 2019, delays in the introduction of new rolling stock meant Northern were given special dispensation to operate them into the middle of 2020. The Class 144s have been spending their last months on lines around Sheffield.
KWVR operations manager Noel Hartley said: "We are storing the Pacers for Northern whilst the crisis is ongoing. There isn't enough space on the national network to store trains when the service is drastically reduced and so we are doing our bit to keep the trains moving in Yorkshire.
"It is helping us too by providing some income for our Worth Saving appeal. As with many other tourist attractions we are desperately trying to raise funds to stay alive. This year is the 50th anniversary of the KWVR appearing in The Railway Children film, something we hoped to celebrate but is now looking more unlikely."
Mr Hartley estimates that the railway, which has charitable status, needs £200,000 in public donations to survive a prolonged shutdown.
To donate to the Worth Saving appeal, click here.What is happening to the Pacers?
The remainder of the Pacers will disappear by August when their leases expire. Many will be taken for scrapping - several have already been dismantled at metal recycling yard C F Booth in Rotherham.
Heritage railways around the country have purchased Pacers to run as preserved locomotives. They include the Wensleydale Railway, who were gifted one of the Class 142s by Pacer enthusiasts Sam and Mim Slatcher. The couple have also bought a Pacer for themselves to convert into an eco-home, and it is currently being restored by Wensleydale Railway volunteers.
The National Railway Museum have also received a Pacer donated to the national collection, and it will run on a test track at their Locomotion site in County Durham.