The RAIB said: “The tram was travelling at around 15mph (25 km/h) at the time of the collision.
“The pedestrians, who were young people, were both injured in the accident, one of them seriously.”
The crossing is south of the junction of Woodbourn Road and Staniforth Road and at the time of the accident, the pedestrians had travelled north along Woodbourn Road before using the crossing.
The RAIB said: “The road junction is protected by road traffic lights and tram signals, while signs and road markings inform crossing users of the presence of trams and instruct them to look both ways.”
It said people approaching the crossing from Woodbourn Road were separated from the tramway by a fence which was about 145ft (44m) long.
The RAIB said its findings, which would be published at the end of its investigation, would include any recommendations to improve safety.