Train conductors on London Midland, which runs trains through Northampton and Long Buckby, are being balloted on strike action.
Members of the rail union RMT said it was balloting members over a decision to place security contractors on London Midland trains.
General secretary of the RMT, Mick Cash, said: “London Midland’s attempts to place external security contractor’s on board trains with an already safety critically trained guard without agreement with RMT is totally unacceptable.
“Over a two-year period RMT negotiators offered various counter proposals to the company which were dismissed out of hand every time or regretfully ignored.
“The imposition of external contractors who have no additional skills or powers of authority to remove unruly passengers or fare dodgers is something we are totally opposed to.
“Our members are fearful that even minor issues will escalate out of control particularly when these security contractors don’t have the appropriate training, skills or knowledge for working within a focused customer service environment like our members do.
“We have been left with no option but to ballot for industrial action to ensure that management finally sit up and take notice of our members’ concerns.
“The union remains available for meaningful and constructive talks.”
In total, 523 RMT members are being balloted. The ballot concludes on February 2.
London Midland has said the security contractors were in addition to senior conductors, have different duties, and that the changes were made following feedback from staff and passengers. The company added that it had no intention of removing conductors from its trains.