Grand Central, part of the Arriva Group, which runs services from London King's Cross to York, the North East, Doncaster and West Yorkshire, said the volunteers would not replace staff, adding that it had run the scheme for more than eight years.
But Mick Whelan, general secretary of Aslef, criticised the company, saying it wanted "unpaid slaves" to provide services.
He said: "It's outrageous that Grand Central expects unpaid volunteers to turn up to work and look after passengers at stations which are unmanned because the company wants to boost its profits by not employing - and properly paying - people to do a job.
"There are important safety and security issues here. It is important, for the safety of passengers and staff, such as train drivers, that stations are manned, and we know that infrastructure can be a target for terrorists."
In a statement, Grand Central said: "Community ambassador volunteers do not replace staff in safety-related roles or any other capacity.
"Grand Central has run a volunteer-based community scheme for over eight years on our North East and West Riding routes to welcome visitors to the area.
"As part of a longstanding commitment to civic partnership, members of the public who are passionate about their community volunteer their time and have won awards for their efforts.
"The scheme is part of Grand Central's deep-rooted commitment to the communities it serves, providing direct train services to London which enable local businesses to thrive and opening up opportunities for leisure travel.
"The company recently celebrated its 10th anniversary and is regularly rated Britain's top train operating company by its customers in the National Rail Passenger Survey and Which? research."