New York is far denser than any other large American city, with an average of 27,000 people per square mile compared with 2,500 to 4,000 for most American cities. Although the city is criss-crossed by an extensive subway system, there are still some neighborhoods that are more than half a mile from a subway station.

So naturally, what those neighborhoods need is an ultra-low-capacity, high-cost form of urban transit: a streetcar. At least, that’s what Mayor Bill de Blasio thinks: last week, he proposed to spend $2.5 billion building a 16-mile streetcar line connecting Brooklyn with Queens.

This is such a dumb idea that even transit advocates oppose it. Streetsblog observes that the proposed streetcar route doesn’t easily connect with subway stations that would give riders access to Manhattan. It also argues that bus-rapid transit (which New York calls “select bus service”) makes a lot more sense than streetcars.

TransitCenter advocate and Brooklyn resident John Orcutt argues that “the American streetcar ‘renaissance’ of the past 15 years has mainly turned out turkeys”: slow (“Reporters for The Oregonian, CharlotteFive and Atlanta magazine have all laced up sneakers and outraced their local streetcars on foot”), expensive (“L.A.’s streetcar has seen its initial cost estimate more than double”), and underperforming (“ridership on Salt Lake City’s S‑Line is less than half of planning projections”).

TransitCenter head David Bragdon, who previously was president of Portland’s Metro Council, agrees. “Most streetcar projects in the U.S. provide slow, unreliable service that does not serve many people,” Bragdon noted, urging New York not to “repeat the mistakes of other places and spend $2.5 billion if the result is not useful transportation for riders.”

While Portland often claims its streetcar is a great success, it has inflated ridership numbers by at least 19 percent and gained most of the ridership it by offering free rides to most passengers for the first dozen years of operation. Even though it supposedly started collecting fares from all riders in 2012, average fare revenues in 2014 were still just 4 cent per trip, showing that no one is enforcing the fare.

TransitCenter also questions de Blasio’s claim that streetcars will generate enough new development to pay for themselves. “Much of the property adjacent to the route is undergoing large-scale development without the spur of a new transit proposal,” says a TransitCenter blog post. “Would more value be realized by supporting transit projects of proven effectiveness in other parts of the city?” In fact, as I’ve repeatedly pointed out, streetcars don’t generate any economic development unless that development gets additional subsidies. Even Portland’s city auditor agrees.

Few of the critics have commented on the high cost of de Blasio’s proposal. Portland spent just under $150 million on its 3.3‑mile Eastside streetcar line, which it said somewhat proudly was the most expensive streetcar line ever built. De Blasio’s line would cost more than $150 million per mile. Labor costs in New York may be somewhat higher than in Portland, but I don’t know of any inherent reason why construction costs should be more than three times as much as elsewhere.

Nor does anyone raise the capacity issue. For safety reasons, a single streetcar line can only support about 20 cars per hour. When jammed full, with most people standing and packed together more closely than most Americans are willing to accept, a streetcar is rated to hold about 134 people, for a throughput of 2,680 people per hour in each direction. By comparison, New York City’s subways can move close to 50,000 people per hour, and buses on city streets with a dedicated lane and parking strip can easily move more than 10,000 people per hour (and nearly double that on double-decker buses), most of them comfortably seated. Plus, if a bus breaks down, others can go around it while if a streetcar breaks down most of the line must shut down as they are built with few passing tracks.

Also little noted is the conflict between bicycles and in-street rails. New York has seen a quintupling in bicycle commuting since 2000, and streetcar tracks are a major hazard to these cyclists. A survey of 1,520 Portland cyclists revealed that two-thirds “have experienced a bike crash on tracks.”

The real purpose of the streetcar is to give the owners of housing projects that are currently under construction along its proposed route a Disneyland-like ride they can use to distinguish their projects from others in the city. They won’t get it very soon, however: de Blasio’s plan calls for construction to begin no sooner than 2019 and completion in 2024. For a lot less money, the city could start a locally branded bus service in a few months that wouldn’t cause as much congestion and wouldn’t create a street hazard for cyclists.

The irony is that de Blasio campaigned for office on the claim that, unlike his predecessors, he wouldn’t cowtow to developers. Now, when the city has far higher transportation priorities elsewhere, he wants to blow $2.5 billion on a toy train that, at best, will slightly enhance the value of developments that are being built anyway and at worst add to congestion and make streets more dangerous for cyclists.