A collision at the Loftus tram museum three years ago highlights the importance of strong risk controls, an investigation has concluded.
No one was injured in the accident involving a runaway tram, but 16 passengers were evacuated from the other tram shortly before they hit.
One tram sustained substantial damage and the other had minor damage.
The Office of Transport Safety conducted an investigation for the Australian Transport Safety Bureau into the accident at Sydney Tramway Museum on Sunday, about 11am on May 15, 2016.
The collision involved an unmanned 675 J class tram and a 1054 Nagasaki tram.
"The Nagasaki was approaching Loftus after completing a tourist operation between Sutherland and Loftus when the crew noticed the unmanned J class moving towards them on the same track," the final report said.
"The crew responded by stopping the Nagasaki and evacuating all 16 passengers safely before the collision occurred."
The report said the J class tram had been parked on a downhill gradient using its air brakes, with a hardwood chock applied under its wheel, when it suddenly rolled away.
"In line with the museum's procedures, the runaway tram's handbrake had not been applied when it was parked and when air released from its brake system, a single hardwood chock was left to restrain the tram's movement," the report said.
"Previously a softwood chock, which would deform and create a tight wedge, would have been placed under the tram's front wheel, but the tram was able to roll over the hardwood chock.
"The investigation highlights the importance of ensuring that any changes to a risk-control process do not reduce that risk control's effectiveness.
The investigation...found that the hardwood chock was newly adopted and its use had not gone through a change management process.
"As a result of the investigation, the museum has taken a number of proactive safety measures, including making the application of handbrakes mandatory, and taking steps to ensure that trams are parked securely on level track.
"The investigation also highlights the importance of ensuring that any changes to a risk-control process do not reduce that risk-control's effectiveness."