That Thursday will have the biggest impact on the normal work-day routine of the CBD, with a combined 405,000 workers and WYD pilgrims expected to congregate for the Pope's official boat-a-cade from Rose Bay, and then green-light motorcade through the city.

George Street will be closed as a central walking route for Catholic faithful, and the strain on public transport will be immense.

The Transport Minister, John Watkins, has publicly urged Sydneysiders to catch the bus and train to work and leave the car behind.

But this morning, the Rail Tram and Bus Union has given notice of a 24-hour strike on that same day, effectively shutting down the CityRail network, in a fight against the Government's 2.5 per cent pay increase cap on the NSW public service.

For months the union and RailCorp management have been at loggerheads over the expired enterprise bargaining agreement that needs updating. Talks broke down last month, and morale among train drivers, guards and station staff is at a low.

"This is not a decision taken lightly - it is one that shows the depth of anger and desperation our members feel about the way Railcorp is treating its workers," said RTBU NSW secretary Nick Lewocki.

"After 11 months of negotiations our members have been forced to breaking point. Today's decision is a reflection of the desperation among rail workers who cannot afford a cut in take home pay, nor are they willing to green-light government cuts to services."

That Thursday will see a huge number of road closures throughout the city, including Hickson Road, George Street, Bridge Street, Phillip Street and Macquarie Street.

Bus terminals at Circular Quay will be relocated to Martin Place, and chronic congestion is expected on the city's outskirts.

The strike has been carefully planned for that date: "The decision to take action at a time when the rail network will be desperately needed sends a signal to the entire community that the network is at breaking point."

Linton Besser is the Herald's Transport Reporter.