Crews have opened a second platform at Union after three years of construction.
TTC spokesperson Brad Ross said the new platform will help ease congestion inside the busy station.
"Customers who are travelling north on the Yonge line toward Finch station will be using the new platform," he said. "Customers travelling north on the University/Spadina line toward Downsview station will use the existing platform."
The new platform cost $137 million. The existing platform, built in 1954, is a centre platform located between the tracks that served both sides of the Yonge line.
Ross said the existing platform was unable to meet current capacity needs at Union Station, which is the third busiest station on the TTC's subway system, handling about 115,000 passengers on a typical weekday.
"It is very crowded during rush hour after an event at the Air Canada Centre or the Rogers Centre," said Ross. "The platform can become crowded almost to the point of becoming unsafe."
Renovation work will now move to the existing platform, which Ross said will receive the same finishing treatments as the new platform. Work on the existing platform, which is expected to conclude in early 2015, will include 150-metre long glass wall that displays artwork.