Sydney's stretched rail network faces another significant test next Monday when trains begin running through a junction at Hornsby in the city's north following a major engineering upgrade, adding to pressure from an expected surge in patronage as more people return to work and the possibility of a driver shortage.

The result of a ballot of about 9000 rail workers on whether to take protected industrial action over a pay dispute is also due to be released on Friday, capping off a horror week for Sydney commuters after the network was thrown into chaos on Monday and Tuesday.

Sydney Trains chief executive Howard Collins said the upgrade at Hornsby where multiple lines intersect had been "very complex" and running trains through it for the first time on Monday would be challenging and put more pressure on resources.

"We don't often do this magnitude of engineering work. It's open heart surgery while running the marathon," he said. "This is all part of the preparation for being able to run three-minute services down the North Shore line to Chatswood."