The first metro journey marks the start of more than 6500 hours of testing which will determine when the line opens to passengers next year.
The dramatic incident occurred on a packed city-bound train during peak hour on Wednesday morning.
The fleet of new Spanish-built passenger trains is already $826 million over budget and on track to be at least three years late entering service.
Commuters also face disruptions from full or partial closures of the busy M1 metro line over several weekends,as well as services stopping earlier some nights.
The globe-like structure will tower 30 metres when it is finished,making it visible to passengers when the first flights start late next year.
A union representing almost 1000 rail workers has been unsuccessful in its appeal to the Federal Court of a months-long suspension of industrial action.
Excavation equipment has now churned through about a million tonnes of rock deep below the north shore,completing two-thirds of the entire project.
The extra costs are aimed at reducing safety risks,the government says.
Weak patronage figures come as the Minns government seeks help from Canberra to close a funding shortfall for the line’s extension.
A delayed upgrade to the western end of the critical link has been boosted by $250 million in federal funding.