Many in Copenhagen may have experienced difficulty arriving on time for work as a result of the union meeting of S-Train drivers, which was scheduled for 7am on Monday, Ritzau writes.
The meeting is a step in ongoing negotiations over a new labour agreement for the regional train drivers.
The drivers are reported to be unhappy with terms currently on offer, while national rail operator DSB considers the meeting an interruption to drivers' normal work in breach of employment terms.
DSB's head of information Tony Bispeskov said early on Monday that the extent of the delays was uncertain.
“Service is very irregular, so it is important for our customers to stay updated via our website and Rejseplanen.dk,” Bispeskov told Ritzau.
“We will update both when we know more about what is happening,” he added.
S-Trains were stationary in a number of places, according to a message posted by DSB on its website on Monday. 350,000 people use the overground local trains daily, according to the operator.
Monday's industrial action comes after a breakdown in negotiations between DSB and rail workers' union Dansk Jernbaneforbund over a new labour agreement.
Train drivers are reported to be upset at DSB's intention to reduce union representation in the negotiation structure from 99 to 29 members, according to an article posted on the union's website on November 2nd.
That will reduce the ability of train drivers to influence labour agreements including in relation to work conditions and holiday, the union argues.
READ MORE: Labour disputes in Denmark