Rail commuters travelling between Sheffield and Doncaster face rush-hour disruption due to a signalling failure.
National Rail said services could be cancelled or delayed for up to 40 minutes, although operator TransPennine Express (TPE) warned that passengers could be delayed for up to an hour.
The disruption, caused by a fault in the system near Meadowhall, is expected to continue until about 10:00 BST.
National Rail said rail replacement transport had been “requested” for TPE passengers travelling between Sheffield and Doncaster.