"The services have been disrupted and there is a significant delay in the operations. We are negotiating with the striking motormen and are hopeful of an early resolution of the issues," a Western Railway spokesperson told PTI.
He did not give give details about the number of services cancelled.
Thousands of commuters were stranded at various stations due to the sudden stir.
The Western Line is the busiest of the three suburban lines in the metropolis and carries on an average of over 3.4 million on its 1,250 services on both directions every day.
Asked what are the unmet demands which triggered the strike without advance notice, the spokesperson said they are internal issues.
Railway sources, however, said the motormen (as drivers of suburban trains are called) have for long been demanding providing co-motormen to help them in the operations besides addressing several safety-related issues.
The Western line operates between Churchgate in the south of the megapolis to Virar on Mumbai's outskirts.
Western line motormen had gone on a flash strike on August 19 last year also demanding reinstatement of a colleague who was to be discharged from his duties.
In 2010, the WR motormen went on strike four times,i.E. in January, February, May and October.