A crane is being brought in to help clear the scene where an out-of-control train had to be deliberately derailed in north-west Tasmania.

Police have secured the site on one of Devonport's main roads where the cement train was stopped in its tracks on Friday morning.

The deliberate derailment of the remotely-controlled locomotive with seven wagons near the wharf area avoided a crash through a city intersection.

When it came to a halt, part of the train crashed through a steel fence, injuring two pedestrians who were treated for minor injuries.

Transport safety investigators are being flown in from interstate to try to piece together exactly what happened.

Devonport Mayor Annette Rockliff said the derailment came as a shock to the community.

"There'll be quite a bit of repair work but in terms of human life and limb we're grateful that it wasn't a lot worse," she said.

"People will have to find alternative ways around the city."

A worker steps over a broken fence where the train came to rest. ( ABC News )

The crane will be used to attempt to recover rolling stock.

A section of the main road into Devonport, Formby Road, between Stewart Street and Best Street, will be closed while the work is undertaken.

TasRail lost control of the train near Railton, about 20 kilometres out of Devonport, but the company said it was unsure how the incident occurred.

TasRail said the train was being operated using on-site remote control technology.

It is unclear how it got out control for nine minutes, travelling at up to 50kph.

A section of Formby Road will remain closed to traffic for the entire weekend. ( ABC News )

It was diverted to a dead-end track and derailed with a permanent derailer — a block placed over one of the rails of the siding.

The union representing the state's rail workers described the incident as "absolutely a one-off".

Rail Tram and Bus Union state secretary Rick Bean said the train controller involved was "reasonably OK given the circumstances".

"It's been running very safely and successfully for many, many years not even a remote indication of any issues such as this," he said.

"We are very grateful and thankful no one was seriously injured."

TasRail said under its management, derailments statewide had reduced from an average of one a month to one a year.

Australian rail safety expert Phillip Barker said trains should generally pass through two safety systems before leaving a loading facility.

Derailers like this one are used to stop out-of-control trains. ( Supplied: IMTRAM PTY LTD )

"The driver or a shunter, someone on the ground, will remotely control the train during the loading process," Mr Barker said.

He said runaway trains usually occurred for one of two reasons; either a failure in communications between the driver and the driverless train, or faulty or uncharged brakes.

"There should be a mechanism that stops the train, so if it loses radio [communications] there is a failsafe device that brings the train to a stop."

If a train is travelling at speed, a block placed on one side of the track at the end of the siding, called a derailer, will knock the locomotive off the tracks.