The agreement yesterday of New Jersey Transit to lead an environmental study of a proposed new cross-Hudson Amtrak tunnel increases the chances that taxpayers are going to get railroaded into a deal where the starting price tag​​ is $20 billion. It would lavish federal and New York tax dollars for the benefit of commuters from but a few counties at New Jersey.

What a boondoggle.

Earlier this month, the federal transportation secretary called the tunnel a “national priority” and announced that red tape for the project would be expedited. That followed a letter to President Obama from Governors Cuomo and Christie promising to cover half the project’s costs – that’s $10 billion. “Our shovels are ready,” they dissemble.

The mega-project is being touted by politicians like Senator Schumer as an urgent necessity to prevent “transportation Armageddon.” That alarmist rhetoric came at the end of a summer of equipment failures in the existing 105-year-old tunnels between New Jersey and Penn Station. The delays were a “soul-chilling premonition of our future,” Mr. Schumer warned, promising he would “try to get the federal government to pay for as much of this as possible.”

Yet the new tunnel is far from necessary and the price way disproportionate to any benefits. By contrast the Tappan Zee Bridge reconstruction is only pegged at $4 billion. The current tunnels, more than a century old and damaged by Sandy, need repair. At some point soon, they will have to be rehabilitated, which would require several months of service disruptions.

There are, though, two Hudson rail tunnels serving Penn Station. At the moment they operate at max capacity during rush hours, carrying 450 trains a day on average. Of those, 100 are Amtrak trains and 350 are operated by New Jersey Transit, mostly for its Midtown Direct service, which moves commuters from the tony suburbs of Northeast Jersey directly to the center of Manhattan.

If the tunnels are repaired one at a time, that would leave room for 225 trains a day. That means Amtrak service would be unaffected. It’s New Jersey Transit that would bear the brunt of the project, with cross-Hudson service temporarily reduced by two-thirds. That’s hardly “transportation Armageddon” (the phrase connotes an end-of-times battle).

Rather, it means that most commuters from New Jersey would need to add perhaps 15 to 20 minutes to their trips to work each way, by switching to a PATH train at Newark or Hoboken, or to a ferry. Last year, 48.6 million New Jersey Transit riders used the Hudson River tunnels to Penn Station. Up to two-thirds, or 32.4 million, would see temporarily longer commutes while the tunnels are repaired.

Fortunately there’s plenty of room for these additional commuters. Last year PATH trains carried 73.7 million commuters across the river. That’s an impressive number, but a far cry from 1927, when the service – then known as the Hudson & Manhattan – moved more than 113 million commuters. Similarly the Hudson River ferries. Last year 8.5 million commuters crossed the river by boat – but a fraction of the 27 million who commuted that way in 1927.

The new Hudson River rail tunnel would, admittedly, make it easier to commute to Manhattan from New Jersey. So why should New York taxpayers be asked to chip in $5 billion for the project? What possible argument can be made for the long-suffering taxpayers of Louisiana and Georgia and California and Alaska to be forced at the point of a gun to cover half the cost?

If anyone should be asked to pay for the cost of the new tunnel, it’s the New Jersey residents themselves, and their communities, who would benefit from it. Midtown Direct service is credited with boosting real estate values in those towns by $11 billion, according to the Regional Plan Association. That’s not to mention the extra $250 million a year in higher real estate tax revenues raked in by those towns.

RPA also projects that building the new tunnel would boost real estate prices in those communities by a further $18 billion – all the more reason why New Jerseyans should be the ones to cover the expense of the project. Or be forced to share their real estate windfall with taxpayers in New York, Louisiana, Alaska, and California and the rest of the states that would be taxed to help them out.

Wouldn’t it be better to sell the tunnels? The Hudson River tunnels we use today, after all, were built more than a century ago not by the government, but by the Pennsylvania Railroad, a private company seeking to increase its profits. Isn’t private enterprise the institution to make the big decisions over whether, and where, to build major infrastructure projects?

Mr. Bennett is a contributing editor of The New York Sun