Sydney’s CBD light rail won’t reach its "wildly optimistic" running targets, according to the transport union, which says its drivers are struggling "under immense pressure" to right the flawed $2.9 billion service.

The union attack comes almost eight years after an Infrastructure NSW report to the O’Farrell government said CBD light rail had "no material speed benefits" and would be "significantly more expensive than bus services".

Sydney's light rail drivers are struggling under immense pressure, the transport union says. Credit:Steven Siewert

The government on Monday said travel times on the Randwick to Circular Quay service would decrease from 45-50 to 40-45 minutes this week, still slower than the 38-40 minutes forecast to NSW Parliament last year.

The Rail, Tram and Bus Union NSW accused the government of trying to shift the blame for the sluggish light rail journey times to its drivers, rather than address fundamental problems with the service.