A peak-hour train split in two with more than a hundred passengers on board last week, after Metro Trains told a driver to ignore warnings that the carriages were about to separate.

In a rare safety incident being probed by rail safety and transport watchdogs, an X'Trapolis train carrying about 150 passengers broke into two parts while travelling to Lilydale early on Friday evening.

Firefighters assist passengers left stranded on carriages that separated from a moving train at Croydon. Credit:Luke Whiteside

The last three carriages of the six-carriage train dislodged and ground to a halt between Croydon and Mooroolbark before 5pm. Passengers were on board but no one was injured.

The driver, who was apparently unaware of the dislodgement at first, continued to drive the train's first three carriages for about 300 metres until he realised what had happened and applied the emergency brakes.