Another major railroad has knocked on the doors of the court to get a ruling on whether it has the authority to require all its employees to get vaccinated against the coronavirus.

BNSF railroad filed a lawsuit on Sunday against its major unions over its vaccine mandate. Before BNSF, Norfolk Southern and Union Pacific have filed similar lawsuits against the unions last month. The unions have filed their own lawsuits in response. The unions argue that the railroads should have negotiated with them before imposing their mandates.

The lawsuit filed by BNSF states that it regularly updates its requirements for workers when federal regulations change, so the vaccine mandate should be no different. BNSF said the unions don’t dispute when it changes drug testing rules, and they didn’t fight it earlier this year when the railroad imposed a mask mandate after the Federal Railroad Administration ordered it.

“BNSF has a long-standing and well-settled past practice of unilaterally changing its rules and policies governing position requirements, medical standards, and safety in response to lawful federal government directives,” the railroad said in its lawsuit, the Associated Press.

The railroads have said that employees will be disciplined if they fail to comply with the vaccine mandate.

The unions have not yet responded in court to BNSF’s lawsuit Monday, but in the other lawsuits, they have argued the railroads were violating the terms of their contract by requiring vaccines and by offering bonuses to workers if they do get vaccinated. Both Union Pacific and Norfolk Southern are offering employees $300 if they get the shots.

“The non-negotiated bonuses and unilateral decision to discipline employees who are not vaccinated functioned to undermine the collectedly-bargained agreements and reduce Norfolk Southern’s obligations under those agreements,” the Sheet Metal, Air, Rail and Transportation Workers — Transportation Division, or SMART-TD, union said in its response to Norfolk Southern, according to AP.

Leaders of the SMART-TD union and the Brotherhood of Locomotive Engineers and Trainmen have said they generally support workers getting vaccinated, but they don’t think the railroads should require it without negotiating first.

The railroads that have imposed vaccine mandates argue that they are complying with President Joe Biden‘s executive order requiring all federal contractors to have their employees vaccinated. The railroads are not alone in facing resistance to vaccine mandates. Similar rules have generated disputes in a variety of workplaces.

Federal judges have yet to rule on any of these lawsuits related to the railroads’ vaccine requirements.

BNSF, Norfolk Southern, and Union Pacific are three of the largest US railroads. A spokeswoman for the fourth major US railroad, CSX, said officials there are still weighing whether to impose a vaccine mandate in response to the executive order but they are encouraging employees to get the shots.

(With AP inputs)