In a statement on SMRT's Facebook page, the train operator said it was informed at 6am that an insulating rail joint between the two stations had been damaged. Such joints provide insulation along the running rail between two track circuits.
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Here is the statement SMRT provided on its Facebook page:
We were informed at 6am this morning that an Insulating Rail Joint (IRJ) between Ang Mo Kio and Bishan MRT stations has been damaged. IRJs provide insulation along the running rail between two track circuits.
SMRT engineers have restored the track circuits to prevent further damage to the IRJ, while the IRJ will be replaced after operation hours tonight.
We wish to assure passengers that we are monitoring the situation very closely, and that there is no congestion at this point in time.
However, please provision an additional 4 minutes of travelling time between Ang Mo Kio and Bishan stations today. Southbound trains (towards Marina Bay) between Ang Mo Kio and Bishan MRT stations are now running at 18kph to ensure system and passenger safety.