Building Intercity trains in County Durham would save the UK economy more than £100m a year, a report by the Northern TUC has found.
A consortium led by Hitachi was the preferred bidder to build carriages and trains at a site at Newton Aycliffe.
However the government has delayed making a final decision until 2011.
The report said savings would be made through a drop in benefits payments and an increase in tax revenues as a result of new jobs being created.
Some campaigners fear the trains could end up being bought from overseas and the Northern TUC is urging the government to consider its figures.
Hitachi was provisionally awarded a £7.5bn contract to build carriages, with its preferred location being Newton Aycliffe.
But the scheme is dependant on Hitachi being granted the Intercity Express Programme Contract.
The Northern TUC's calculations are that more than £100m a year would be saved through the creation of 800 direct jobs and 7,500 supply chain jobs.
It said that figure was based on the taxpayer gaining £13,000, as a result of reduced benefit payment and taxes paid, for every £27,000 job created at Newton Aycliffe.
Northern TUC policy and campaigns officer Neil Foster said: "We have long argued how the North East people and economy would substantially benefit from Hitachi's presence at Newton Aycliffe.
"These new figures illustrate how taxpayers up and down the country will also benefit if this programme goes ahead and the trains are manufactured here."