Eight people were injured, two of them needing hospital treatment, but they have now been discharged.
The MTR says repair work is almost finished and both East Rail Line platforms have resumed services at Hung Hom Station.
CEO Jacob Kam was asked whether employees could have caused the incident.
He said: “At this point we are not ruling out any possible causes, although up to this point, we have not discovered, for example, any external objects that may have caused the incident. But we continue our investigation to cover all aspects.
“We have established an investigation panel, consisting of both local and – of course – international experts.
“We will look into every detail – every possible cause of the incident – and I’m sure we will improve from that.”
The company has been plagued a series of operational problems over recent years, including service delays and in March another derailment, when two trains collided near Central while carrying out testing.