The Delhi Metro Rail Corporation on Wednesday suspended four employees, a day after an unmanned train on trial run broke through the boundary wall at Kalindi Kunj depot in South Delhi.

Tuesday’s mishap occurred just days before Prime Minister Narendra Modi is scheduled to inaugurate the Delhi Metro’s Kalkaji Mandir-Botanical Garden line on Christmas Day. Unmanned trains will ply on the route in about two to three years.

The metro authority said the accident occurred because the maintenance team did not check the train’s brakes before the trial run. The corporation ordered a high-level inquiry into the incident, saying a committee of three of its officers will look into it.

The Delhi Metro Rail Corporation said the train was being operated manually when it was taken to the Kalindi Kunj depot to be washed. When the operator moved the train up a ramp, it slid backwards because the brakes were not functioning.