Toronto's transit system will receive 39 kilometres of new subway and light rail track in a $12-billion expansion package announced by the province and city on Thursday.

The details of a multi-billion dollar transit expansion were outlined by Ontario Premier Dalton McGuinty and Mayor Rob Ford on Thursday, with both men stressing the importance of getting construction underway as soon as possible.

McGuinty said the province would invest $8.4 billion to build an underground light rapid transit line along Eglinton Avenue, spanning 25 kilometres from Black Creek Drive to Kennedy station.

The line will also replace the Scarborough Rapid Transit line, running aboveground from Kennedy Station to Scarborough Town Centre.

"This is the biggest ever investment in Ontario on public transit. This transit line will be the spine of the Toronto and the regional systems," McGuinty told reporters on Thursday.

He added the provincial investment will cover the cost of 130 new LRT transit cars, which will be built in Thunder Bay, Ont.

Meantime, the city will move ahead with an extension to the Sheppard subway line at a cost of approximately $4 billion.

The subway extension will run 14 kilometres from Downsview subway station to the Scarborough Town Centre.

Ford was not prepared to outline how the city will pay for the expansion on Thursday, but said the majority of the cost will come from the private sector, stressing that the city would own the line after construction was complete.

He added that the city will ask the federal government to use a previous $330 million commitment to fund the Sheppard project, and that the province may provide more money to cover part of the cost.

McGuinty said if the province comes in under the $8.4 billion budget on its project, the savings would be dedicated to the Sheppard line.

Ford has received some criticism for excluding the Finch Avenue West neighbourhood from his transit plan. The low-income neighbourhood had been promised a rapid transit line under former-mayor David Miller's Transit City plan.

Ford promised on Thursday that the neighbourhood would have a subway line within ten years.

"We will make major improvements to the bus service along Finch Avenue West until we can introduce rapid transit there as well," he said.

Construction of the Eglinton rapid transit line and the Sheppard subway extension should be complete by 2019.