If you're one of the 200,000 commuters who flow through Union Station as part of your typical weekday commute, prepare for some changes to your routine today.

The Bay Street concourse, located on the east side of Union, is now closed as a renovation expected to take about two years gets underway. The closure is part of a massive renovation of Union Station, which is Canada's busiest transit hub.

Metrolinx spokesperson Anne Marie Aikins said there will be about 100 extra staff on hand today to help commuters find their way. Most will be rerouted through the recently renovated York Street concourse.

About 100 vest-wearing Metrolinx employees were on staff this morning to help commuters find their way, now that the Bay Street concourse is closed. (Trevor Dunn/CBC)

"It's just a matter of people finding a different route. Give yourself a few more minutes," she said on Metro Morning Monday. "This is the last big piece [of the renovation]. We purposely held it back until after the Pan Am Games. But we need to update it and prepare for the growth of the future."

Aikins said the concourse was last renovated in 1979.

"It is well in need of a repair. It's a massive undertaking; a real feat of engineering."

Crews will dig down two levels below the existing concourse.

The renovations will:

Improve the concourse's link to the TTC. Passengers will no longer have to go outside to connect between the subway and GO Transit.

The floor space will expand by about one third. The existing concourse was often crowded during rush hour.

Better access to the PATH system.

New shops and restaurants.

"It will be bright and open, much easier for commuters to get through and it will fit a lot more people. We really need that," said Aikins.

Confused commuters should look for Metrolinx staff members wearing vests.

"We'll get them to their destination," said Aikins.

Below is a selection of how the closure has been received on social media.