The rail firm announced last week plans to close its site on Strand Road by next year.
Around 180 staff are facing uncertainty over whether they are offered transfers to an Alstom unit in Widnes.
And the building’s owners have said they are assessing options for the site.
Unite, which is representing a majority of those affected, said some may also be offered a transfer to work for Wincanton in Blackburn.
This follows on from Preston MP Mark Hendrick calling for action by the Prime Minister with this blow coming weeks after BAE Systems’ jobs cut announcement.
A spokesman for Unite told the Post: “We have sadly accepted that the factory will close. We are currently in the process of getting the best possible deal for our members with regards to voluntary redundancy and the opportunity for workers to relocate to the new site at Widnes.
“Unite is also in the process of negotiating TUPE (job transfer) terms for workers involved in stores as this sector will be transferred to Wincanton at its Blackburn premises.
“The union has further meetings scheduled at the company this week.”
Alstom vacating the historic factory close to Preston Docks places the future of the site in doubt.
Owners, property investment group St Modwen, has said it is “currently reviewing the options for the future of the site.”
Alstom said plans to “centralise” their services had led to the decision to vacate its Preston base to the “world class facility” in Widnes.
A statement added: “Consultation is ongoing with the trade unions and staff and it would be inappropriate to comment any further.”