A state proposal to build out several missing links in an existing waterfront trail from South Boston to the Blue Hills has been sent to Washington D.C. authorities— whose decision on whether to fund the project will likely make or break the long-awaited project.

The state’s Department of Conservation and Recreation (DCR) filed its 26 page application on Monday— the deadline day for the latest round of so-called TIGER grants issued by the U.S. Department of Transportation. DCR hopes that the federal agency will award the state some $13 million to complete the Neponset Greenway corridor, which stretches from Commercial Point in Dorchester to the Blue Hills in Milton. The exisitng multi-use trail along the Neponset River and Dorchester Bay is already a hit with bicyclists, walkers and families— but there remain some key stretches of the trail that have never been completed in Milton, Mattapan and Dorchester.

After a thorough public process that has taken over two years to perfect, the DCR briefed Greenway advocates and abutters on their plans this summer and built consensus on how and where to route the trail links. Now, with a total budget of just over $17.5 million needed to actually construct the trail, the state hopes the federal dollars will pay for up to 75 percent of the project’s cost.

If the funding is secured, two existing sections of the Greenway Corridor will be lengthened and improved with construction jobs that could start as soon as next year. And a new middle section will be added to “fill the gap” between Mattapan square and Central Avenue, where the existing Greenway trail abruptly ends.

“There are so many great aspects to finishing the 9 mile Greenway,” said DCR Commissioner Edward M. Lambert, Jr. “It really is all about connections when you consider how we’ll connect the Harbor Walk in South Boston and Columbia Point to the Blue Hills. We’ll be connecting to mass transit with the T stops, UMass Boston, and connecting communities in Mattapan and Milton. There are so many great aspects to it and it’s a strong application that we hope reviewers will recognize as an incredible enhancement.”

One prominent feature of the trail’s expansion is a segment that will allow Greenway users to bypass a hazardous I-93 expressway off-ramp at Morrissey Boulevard by building a new raised boardwalk through property now controlled by National Grid next to the landmark Dorchester gas tank on Commercial Point. National Grid has agreed to donate the land easement that will make the construction of the boardwalk possible, a savings of some $378,000, according to DCR.

“The boardwalk is dynamic and will provide a great vista,” said Lambert, who said that—if funded— the Commercial Point connection could be in progress by next winter with a anticipated completion date of the end of 2013. The new 1.75 mile segment would run next to a $9.4 million solar array that is now being built with private funds next to the National Grid gas tank and provide safe passage for bicyclists and pedestrians to pass under the expressway on a southern course towards Tenean Beach, Port Norfolk and Pope John Paul II Park. Greenway users would still need to walk along an access road between Victory Road and Tenean Beach, but would be able to bypass the off-ramp at Morrissey Boulevard which presents a formidable obstacle right now.

The total cost of the completing the Greenway, according to the application filed Monday, is $17,557,474 of which the state hopes to secure $13,175,680 through the federal grant award. An additional $3.3 million would be allocated through the DCR. The state’sDepartment of Transportation would contribute $313,500 in right of way donations, the MBTA would shoulder $196,500 by donating a right of way easment at Mattapan’s T station and Stop & Shop Supermarket Company has promised to contribute $175,000 to help build out a segment at the southern-most portion of the trail in Hyde Park, near its store on Truman Parkway near the Martini Shell Park. This new segment of the Greenway— the smallest in size and budget— would be built-out first to improve access and safety between Mattapan Square and Hyde Park. Lambert said that the shovel-ready segment could be put out to bid within weeks of a decision from the U.S. Department of Transportation, which is expected to announce its grant winners by April 2012.

The most expensive segment of the planned expansion will be the $10 million connection between Mattapan Square and Central Ave. near Lower Mills, where the current Greenway trail ends today. This so-called “middle segment” would run for approximately 1.5 miles on both the Milton and Mattapan sides of the river, much of it along the path of a now-defunct and heavily overgrown railway bed. The proposal calls for the construction of a new bridge over the river to connect the two communities and a “flyover” canopy bridge to avoid trolley traffic at Mattapan Station. DCR officials say that this would be the final piece of the Greenway to be completed if fully funded, with an estimated completion date of June 2014.

Valerie Burns, president of the Boston Natural Areas Network and a leading advocate for the Greenway’s expansion, called the application “pretty extraordinary.”

“The DCR really dug in and put together the strongest application possible,” said Burns. “And by putting it all up on the website and sharing it with everyone, I can’t remember another proposal like this that’s been so publicly shared so that we’re all involved in it.”

For more details on the DCR application for federal funding for the Neponset Greenway, visit their website.