Toronto transit planners may have to look skyward when it comes to carrying the planned Eglinton transit line across the Don Valley.

Leading up to his election last fall, Toronto Mayor Rob Ford vowed to build subways instead of a series of light-rail surface routes in the now-scrapped Transit City plan.

It’s not clear how the Eglinton line, which will stretch from either Jane or Keele Street in the west and continue to Scarborough Town Centre in the east, will cross the Don Valley.

TTC Chair Karen Stintz admits that a tunnel may not be practical under the Don Valley.

“We are still working through the issues of how it is that we have an LRT that is not at grade in a valley,” she said.

Metrolinx executive Jack Collins said tunnelling under the Don would be an expensive undertaking, but not impossible.

“In the middle of the Don Valley you have the Don Valley Parkway so you've got to go down and up and back down and up again across the valley,” he said.

Collins told CBC News a bridge over the Don is one option under discussion.

Either option – tunnel or bridge – would have to be factored into the existing environmental assessment, which could cause possible cost overruns and delays.

Both Stintz and Collins insist the Eglinton line will still be completed by late 2020.