Police are investigating damage on a stretch of railway track approximately 3 miles west of Stonehaven at Auquhirie.
The damage was brought to the attention of British Transport Police at around 1am by a train driver.
Many rush hour trains have been cancelled or disrupted and delays are likely to contine.
Limited replacement buses will operate from Laurencekirk to Aberdeen for trains terminating there, passengers can also alight at Montrose for a local bus service to Aberdeen.
Richard Baker, MSP for North East Scotland and Shadow Cabinet Secretary for Capital Investment and Infrastructure said “I am sure both Network Rail and British transport Police will already be investigating the cause of the damage of to the tracks near Stonehaven, but given the impact this incident has had it will be important to have answers as soon as possible. Passengers travelling at the most busy time for our rail services into Aberdeen have had the misery of cancelled and delayed journeys, and many local businesses will have had their day disrupted too.
“While the disruption itself is of huge concern there were clearly potential implications for passenger safety and that is why it is important that the damage appears to have been reported promptly.
“If this has been a case of deliberate damage to the track then it raises issues of track security and the need to identify those responsible. If there has been another reason for the damage this will raise questions for Network Rail to address.”