Older locomotives can also continue to run on city tracks. But V/Line is rapidly running short of trains to run on any of its lines because of a sudden spate of wheel damage that is forcing the operator to withdraw many of the newest trains in its fleet from service. Metro's Stony Point line has been replaced by buses after V/Line took back the line's diesel trains to plug some of the gaps in its timetable. Passengers on all regional lines have been severely disrupted this week, with V/Line scrambling to patch together a skeleton train timetable backed up by scores of replacement coaches.

The cancellations and disruptions to services began in earnest on Wednesday and hit crisis level on Friday evening, when Metro's safety-related ban forced V/Line to terminate its trains at suburban fringe stations in Sunbury, Craigieburn and Pakenham. Passengers were ferried onto buses or Metro trains. Some services were cancelled entirely. Metro made the emergency decision after a V/Line train on the Dandenong line failed to trigger boom gates and warning signals while crossing Progress Road, just east of the South Gippsland Highway. A Public Transport Victoria spokesman said V/Line trains had stopped running on parts of Melbourne's network while technical investigations were carried out. "VLocity trains continue to operate safely on the regional network," he said.

"This decision is about ensuring the safety of rail services for passengers and the community." Customers were advised to check the V/Line website for updates. It is not the first time a V/Line train has had this potentially deadly problem and the recurrent issue has prompted lasting bans on shorter trains on some lines. This is because the shorter trains are prone to failure to trigger the track circuits that activate boom gates. V/Line has broken up many of its trains into shorter units this week as a strategy to run as many services as possible while other carriages are withheld to repair the growing number of wheel faults.

By late Friday, almost half of V/Line's 59 three-car VLocity trains were out of service for urgent maintenance. Some of the trains are just months old, and were built as part of an ongoing order that is costing the state hundreds of millions of dollars. V/Line insists it has not yet identified the cause of the sudden wear on the inner wheels, which could result in a train derailment if left unfixed. The problem emerged in the past 30 days and a resolution is still being sought. One possible source of the wear being investigated is the new tracks on the $3.65 billion Regional Rail Link between Southern Cross Station and West Werribee, which fully opened in June. A rail flyover between Southern Cross Station and North Melbourne has been pinpointed as a potential source of the excessive wear. The flyover has tight curves that could place pressure on the inner wheel, called the flange. The flyover was not part of the original design for the Regional Rail Link but was introduced to save money.

V/Line chief executive Theo Taifalos​ told The Age this week that the decision to confine much of the VLocity fleet into maintenance yards was his, and was based on safety. Mr Taifalos also said there was no prospect of the entire VLocity fleet being withdrawn from service and that the wheel wear issue was being managed. But Metro's decision leaves V/Line in a state of crisis that threatens to take a severe toll on regional Victorians' travel needs. Loading A V/Line spokesman said the regional operator was notifying its passengers of all service changes within the metropolitan area.