A passenger on board the train posted to social media saying the train was being held at the station for some time after the crash. Train staff were in shock, she said.
Trains have been delayed and some have been cancelled.
Transdev spokesperson Emily Liddell said the line was closed, and their focus was now on looking after the train crew and passengers.
"We have bus replacements running between Taita station and Upper Hutt. It looks like that will continue through peak travel this afternoon.
"Obviously we're looking after the passengers and crew in this situation.
"When there's a serious incident on the track, we'll always relieve our train crew. We'll put support in place for them, if needed.
"The emergency services are looking after the incident itself, but we'll have a full investigation from our perspective of what happened."