Mayor Bill de Blasio's plan to build thousands of affordable apartments over the railroad tracks at Sunnyside Yards is expected to come with a hefty price tag, experts say. View Full Caption DNAinfo/Ben Fractenberg

SUNNYSIDE — Mayor Bill de Blasio's plan to build thousands of affordable apartments over the railroad tracks at Sunnyside Yards might not make it out of the station because of a hefty expected price tag, experts say.

The mayor proposed building some 11,250 affordable units at the rail yards during his State of the City speech earlier this month. And while experts say it can be done — pointing to projects like Atlantic Yards and Hudson Yards — they predict it will be costly and time-consuming.

"We sort of learned how to do them in the modern era, but they're expensive," said Barry Hersh, a clinical associate professor at New York University's Schack Institute of Real Estate.

He compared the mayor's Sunnyside plan to the Hudson Yards project being built on Manhattan's west side, where a platform over the railroad tracks is expected to cost $750 million, according to published reports.

It was not clear how much the platform and development in Sunnyside would cost.

"It's an expensive way to buy land," Hersh said. "If you spend a lot of money to build a platform over the yard, you need to build a lot of units to justify that. You need to have a dense project."

Sunnyside Yards is in a "great" location in terms of accessibility, Hersh said. And since the site is partly owned by the MTA — some acres of which the city has air rights to — it makes it a more attractive development prospect than if it were entirely owned by private parties.

Building big is the way de Blasio will be able to reach his affordable housing goals, Hersh added.

"In order to produce the 200,000 units that the mayor talked about building or saving, you’ve got to do big projects," he said.

The site's 200-acre size is both a draw and a challenge, according to Christopher Jones, vice president of research with Regional Plan Association, who said they were "very glad to see the mayor put this on the table."

"There's almost no place left in New York City where you can look at an area that large, that could potentially support so many of the things the city needs," he said. "The challenge, of course, is that in order to build over an active rail yard of that size, it is going to be incredibly expensive."

It could take years, likely more than a decade, for a project of this size to get off the ground, experts said.

"The history of large development projects in New York is they have to go through several iterations before they're actually built," Jones said. "That’s why its really important to get this discussion going now."

The mayor's office will soon launch a feasibility study to assess the costs and infrastructure needs that developing Sunnyside Yards would require, a spokesman said.

The MTA declined to comment. Amtrak, which owns much of the yards, did not immediately respond to a request for comment.

Jerilyn Perine, executive director of the Citizens Housing Planning Council, said studying the rail yards for housing makes sense.

"I'm not saying that it's not complicated — I'm sure there's many, many technical issues, of course, but let's take a hard look at them," she said, adding that the city has successfully built projects of this magnitude before.

"We have taken on these kinds of big challenges of reshaping our infrastructure in different ways," she said. "We should just remind ourselves that it is possible."