It has been a nightmare week for the South Morang line, with two derailments and multiple track closures in the space of a few days.
The line in Melbourne's north was shut down for most of Thursday because of a maintenance machine's derailment, but normal services had only resumed for a few hours on Friday when another fault led to the line being suspended again.
A defective train ground to a halt at Ruthven station just after midday on Friday, forcing Metro Trains to organise replacement buses for the second day in a row.
A Metro Trains spokeswoman said it was not yet known how long it would take crews to repair the damage.
Photos show that the arm of the top of the train that connects to the overhead power lines is damaged, and is stuck in a downwards position.
But that train got off lightly compared to the train carrying 120 passengers that derailed near Rushall station in Fitzroy North on Saturday.
One passenger was injured in the incident, and that train has been impounded while investigators figure out why it came off the tracks.
It was at the same spot that a track maintenance machine derailed late on Wednesday night, leading to the line shutdown on Thursday.
The rail safety watchdog will investigate the two derailments, which occurred at the sharpest bend on the metropolitan rail network.