With a booming metro population and an increasing reliance on public transport, Toronto needs to expand the existing subway system, says Coun. Michael Thompson.

"What we do know about the city of Toronto, is growth is going to take place," he told CTV Toronto's Janice Golding.

"The question is, will we be able to deal with it?"

Currently, the TTC reports about 470 million riders every year. And that number keeps growing.

That's why Thompson has unearthed a decades-old plan for a so-called "downtown relief line," which would increase capacity on the subway by several thousand each day.

The downtown relief line would look something like this: imagine a broad u-shaped line anchored at Union Station, extending north to Pape on the east and Dundas West on the city's west side.

An older study on the line calculated that the new line would increase subway capacity by 17,000 riders each hour during peak times.

The line would also shorten riding times in neighbourhoods like Riverdale, Leslieville, the Distillery, Queen West and Parkdale. It could also ease traffic on Queen Street and elsewhere.

TTC chair Adam Giambrone recently said that the city would pony up $5 million on a feasibility study for the 13-kilometre line.

Now for the bad news: the price tag for the new line could be anywhere from $3 to $6 billion dollars.

That hefty cost forced the city to abandon the idea in the 1980s.

Still, as the Transit City program begins and work gets underway to extend the Yonge-University line to Vaughan, Thompson says the city can't afford not to expand the TTC.