Mobility is the key to revitalizing Long Island’s economy: mobility within Long Island as well as to and from New York City and beyond. That includes far better reverse commuting — to Long Island in the morning and back to New York City at night.

Despite being the birthplace of the magnetic levitation train, which just set a world speed record in Japan at 375 miles per hour, Long Island is served in the 21st century by a railroad whose system of tracks is essentially the same as it was when it was laid out in the 19th century. Long Island’s population was 50,000 then; it’s now 3 million.

If we didn’t have the Long Island Rail Road, we would need 10 new traffic lanes for cars, and the impact on the environment would be extraordinary. Yet the system primarily allows for traditional commuting into the city — not reverse commuting. That’s no longer sufficient for a 21st-century economy.

The two major railroad projects that are now under construction, the Double Track from Farmingdale to Ronkonkoma and East Side Access, will increase the frequency of trains on the Ronkonkoma branch as well as direct access to the east side of New York City. But to increase mobility and reverse commuting to the extent needed to rejuvenate the local economy, the “Third Track” — or “Fast Track,” as I prefer to call it — is essential.

That means adding a track to the 9.8-mile segment of the Long Island Rail Road’s Main Line from Floral Park to Hicksville. The extra track would eliminate the current bottleneck where so many branches converge. It would increase the frequency of trains and enable effective reverse commuting to take place; it would enhance reliability across the system, and it would permit the electrification of the Port Jefferson, Oyster Bay, and Mastic-Shirley lines. It would thereby benefit the entire region.

For reverse commuting, it would dramatically reduce the average minutes between trains during the period from 6 a.m. to 10 a.m. on weekday mornings. The time between trains from Manhattan would drop from 60 minutes to 16 minutes to Westbury; from 60 to 23 to Brentwood; from 48 to 21 to Ronkonkoma; from 40 to 19 to Huntington; and from 22 to 9 to Hicksville.

Changes of that magnitude would put Long Island’s reverse commuting on a level with White Plains, which is crucial to attracting the jobs we need. Not enough jobs will locate on Long Island, when missing a train means waiting as much as an hour for the next one.

By 2035, within 10 years of its completion, the Fast Track will have generated a projected 14,000 new jobs and $5.6 billion in additional Gross Regional Product. It would also have attracted 35,400 new residents to Long Island — 39 percent of whom would be in the 25- to 44-year-old age cohort, compared with only 20 percent of Long Island’s total forecasted 2035 population without the Fast Track.

The project will have trade-offs, as all infrastructure improvements do. A number of Nassau County communities will endure disruption, inconvenience, and construction in order for the entire region, including those communities, to benefit.

That reality will need to be addressed fairly and respectfully. Ways to mitigate the intrusion must be discussed, and good plans developed.

But not building the Fast Track will harm all Long Islanders, who will continue to lack needed jobs and watch their children and grandchildren leave to find those jobs elsewhere — including in other nearby suburbs of New York City.

An initial investment of $1.1 billion will be required to construct the Fast Track, but the return on investment will be extraordinary. Long Island’s Gross Regional Product will increase by an estimated $67.9 billion in 2050. That’s a return on investment that would make Wall Street envious.

Long Island has centers of excellence and innovation that would make any region proud. They include major institutions — Brookhaven National Laboratory, Cold Spring Harbor Laboratory, North Shore-LIJ Health System, and Stony Brook University, among many others — as well as numerous startups that are spinning off the research and insights of Long Islanders.

What we need now is to better connect Long Island’s innovative economy to that of New York City and beyond — to provide the mobility needed to make Long Island more accessible than ever. The Third Track or Fast Track is the key to unlocking the economic growth for which Long Islanders are longing.