Emergency services were called to the derailment at a busy Haymarket intersection about 10.40am on Monday.
Four firefighters were in the truck and responding to an emergency when the crash occurred, Fire and Rescue NSW told AAP on Monday.
Paramedics assessed several light rail passengers at the scene near Central Station, and a woman in her 70s who suffered chest injuries was taken to St Vincent's Hospital in a stable condition.
A firefighter in his 40s suffered minor injuries and was taken to Royal Prince Alfred Hospital.
"This occurred in a really busy part of the CBD with a lot of pedestrians so we're thankful that no one was seriously injured," NSW Ambulance Inspector Ben Saywell said.
Traffic delays are expected and drivers being urged to avoid the area.
Two of three northbound lanes on Pitt Street have reopened after the street was previously closed.
Pitt St remains closed in the southbound direction between Eddy Avenue and Railway Square.
Part of Eddy Ave also remains closed between Elizabeth and Pitt streets.
The L2 and L3 light rail services also remained suspended between Central and Circular Quay, but are still running between Central and Randwick, and Central and Juniors Kingsford.
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