Loaded with tonnes of cement and travelling at about 50 km/h, multiple carriages were damaged as they left the tracks on the north Tasmanian city's waterfront just after 9am on Friday.

The TasRail freight train struck nearby fences, sending debris flying, which hit and injured two passers by.

Police Inspector Stuart Wilkinson explained how emergency service crews desperately tried to warn the public about the runaway train.

"We had all our police vehicles deployed with lights and sirens. That obviously indicated to people there was an issue (but) they wouldn't have been aware the train was going to derail basically in front of them," he told reporters.

"Given the timing, it was very difficult to get people in place and clear that area.

"It wasn't clear to us where the train would derail, only that ... it would, and it would be around the area of the wharf."

No one was on board the train which was being operated by remote control from a loading yard - a method that has been used for more than 15 years at Devonport.

It was out of control for only a short time after becoming "unsteady" near the Devonport silos, minutes before the derailment, Insp Wilkinson said.

"The train hit a derailment mechanism, and jumped into the air and it came to a stop very quickly and the procedure was very effective.

"Unfortunately it did come across about a 30m stretch of walking track ... and unfortunately there were two people at that spot."

Pieces of fencing hit a woman, aged in her 20s, in the head and a man in his 40s has a suspected broken limb and cuts, police said. They have since been released from hospital.

"We're very fortunate they're not seriously injured," Insp Wilkinson said.

"If the train had gone on, that potentially could have been far more serious."

TasRail said police were notified within three minutes of them learning the train was out of control.

"The train was diverted to a dead end siding track that has a permanent derailer, ensuring the train could be halted in the safest possible location," the operator said in a statement.

The Australian Transport Safety Bureau will investigate why the train lost control.

"The ATSB's safety investigation will include an examination of the locomotive and wagons, interview witnesses, and obtain any available recorded data for analysis, amongst other activities," a spokesman said.

A crane is at the crash scene and the train's recovery is expected to start on Saturday.

Australian Associated Press