Commuters are facing severe journey delays after Cross City North services were suspended between Birmingham New Street and Lichfield.
West Midlands Railway (WMR) said the disruption was caused by a failure in the signaling system between Four Oaks and Lichfield Trent Valley.
Trains running through the area are delayed in both directions as engineers work to restore the service.
A number of replacement buses have been set up to take passengers from Sutton Coldfield and Lichfield Trent Valley.
The rail operator said the disruptions were expected to last until 12:00 GMT and passengers were advised to postpone their journeys.
Anyone who is more than 15 minutes late may be entitled to late repayment compensation, WMR added.
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