A piece of land and infrastructure may have multiple uses. Land might be needed for urban development or for a highway. A two-track structure might be needed for freight or passenger service. A right-of-way might be needed for multiple kinds of rail, or a road, or a power line easement, or a park. In all cases, the correct policy choice is to allocate the land to the use that has the highest social value, and this use depends on the situation at hand. It should not be allocated to whatever one fancies.
Concretely, let us consider the following cases:
1. The High Line. Occasionally, railfans grumble about the linear park, and say it should’ve had passenger rail service instead; read the comments on Ben Kabak’s post on linear parks, or New York City subway forums. But in reality, the High Line is very useful as a park in a busy neighborhood that doesn’t have other parks. In contrast, it’s nearly worthless as a transit line: it’s parallel to a north-south subway that’s operating well below capacity, it would be nightmarishly difficult to connect to any existing line, and the only east-west service it could possibly be useful for is connecting to 14th Street, not the most important job destination in the city.
2. The Northeast Corridor in Rhode Island, south of Providence. The expansion of MBTA commuter rail southward into sprawling exurbs is a major failure of regional transportation policy. Providence is not all that congested by the standards of the larger Northeastern cities; auto-oriented commuter rail toward it is doomed to fail, and near-downtown parking is cheap and plentiful. (The commute market from Warwick and Wickford Junction to Boston is trivial.) In contrast, the line is perfect for intercity service, since it has relatively gentle curves outside city limits, and is straight south of East Greenwich. The South County project not only costs $200,000 per weekday rider, but also makes poor use of high-speed track. Since the line is more important as high-speed rail than as a commuter line, Amtrak should be more aggressive about demanding that commuter projects create their own capacity.
3. The Northeast Corridor in Maryland, north of Baltimore. For the same reasons as the MBTA extension’s eventual failure, MARC underperforms north of Baltimore. Although the line has extensive three- and four-track segments, the bridges are two-tracked, and high-speed rail should again be given priority, including canceling commuter rail if necessary. Ironically, because of more extensive four-tracking, the need for bypasses around Wilmington and perhaps North East, and the at-grade track layout, Perryville is quite easy to connect to Philadelphia by commuter rail without interfering with intercity rail.
4. Caltrain to San Jose, the MBTA to Providence, MARC to Baltimore. In contrast with the situation in points #2-3, those three lines are all useful commuter lines; they are all similar in that they connect two distinct cities that share suburbs, with a rump extension that exists purely for show (into Gilroy, Perryville, and soon to be Wickford Junction). Any and all high-speed rail use of these corridors should permit a reasonable frequency of commuter trains, with timed overtakes when possible and full four-tracking otherwise. On Caltrain, in particular, interference with commuter rail is one reason why the chosen Pacheco Pass alignment is inferior to the Altamont alignment.
5. The Lower Montauk Line. Despite perennial railfan desires (and an empty Bloomberg campaign promise, since scrubbed from his campaign website) to restore passenger service, there’s not much point in regional rail that stub-ends in Long Island City. To give an idea how much demand there is, the LIRR currently runs 5 trains per day per direction into Long Island City. Thus, the line is more useful for freight trains than for passenger trains. This will change if, and only if, there is a way to connect the line to Manhattan through the existing LIRR tunnels, or perhaps new tunnels, but then the cost is going to be orders of magnitude higher than just restoring service.
6. Urban freeways, e.g. the BQE. American freeways were built at a time when, even more so than today, land was allocated based on political power rather than any sort of social consensus or market pricing concept. While Japanese cities have to make do with 4-lane freeways due to high land costs and strong property rights protections, American cities demolished entire neighborhoods to make room for freeways with wide exclusion zones around them. The land occupied by some would be more useful for additional neighborhood housing growth than it is for a freeway. For example, the BQE hogs prime real estate in Williamsburg, right next to the under-capacity Marcy Avenue subway station, and to a lesser extent in the rest of Brooklyn and Queens, and this land could be used for high-density development instead.