LONDON (Reuters) - Rail workers in northern England joined London colleagues in strike action on Monday as a dispute over staffing levels on trains deepened, causing travel disruption in some of England’s biggest cities.

Workers on Southern Rail, a busy London commuter line, took their thirtieth day of industrial action in nearly a year. The RMT union is dispute with the operator, Govia Thameslink Railway, owned by Go-Ahead Group plc, over who should open train doors at stations.

There has been substantial disruption to Southern’s services for months, paralyzing services on busy lines linking London to southeast England. Southern Rail did not say what impact Monday’s strike was having but said that previous strikes had failed to prevent most of their services operating.

The RMT was also taking industrial action on Merseyrail and Northern services, which serve cities such as Liverpool, Leeds and Manchester, also over staffing numbers.