This post partially responds to “The Altamont of X” comments made by Adirondacker, though it is far more general than that.
Whenever a route has to connect three non-collinear cities, compromises must be made between cost and directness. The two basic configurations are a triangle and a Y or T; a triangle is more direct but requires more infrastructure, whereas a Y is the opposite. The purest example of this issue is in Texas; the Interstates connecting Houston, Dallas, and San Antonio form a triangle, but with future high-speed rail, either configuration and many compromises in between are possible. Since not even in Texas is there a pure triangle with equal vertices and nothing in between, each site has its own questions regarding phasing, constructibility, intermediate cities, and relative importance of the triangle’s three sides.
In California, the Altamont vs. Pacheco debate is at least in part a Y vs. triangle debate. Here, the three nodes are Southern California, the Bay Area, and Sacramento. The LA-Sacramento leg is the simplest, because the line would just run straight up the Central Valley. The question is then what to do with the other two. The Pacheco alternative is essentially a triangle: San Francisco-Sacramento service gets an Altamont overlay, or maybe a heavily upgraded Capitol Corridor, and there is wide separation between the Central Valley-Bay Area connection used by trains heading to Los Angeles and ones heading to Sacramento. Altamont is a Y whose branch point is Manteca, with tracks going west to the Bay Area, north to Sacramento, or south to Los Angeles.
The particular case of California, however, favors the Y over the triangle. LA-SF and SF-Sacramento are both important corridors, so being able to serve both more easily is an advantage. Although Pacheco is shorter in distance than Altamont, it is not shorter in time to San Francisco, because more of Altamont is in the Central Valley and less is on the Caltrain corridor; for the same reason, the two options are about even on the cost of LA-SF alone. Altamont is actually a bit cheaper according to the original alternatives analysis, and the recent cost overrun is disproportionately in areas used only by Pacheco, such as the pass itself and the San Jose Diridon complex. Although Altamont has to cross water, a water tunnel parallel to the potential crossing site is currently under construction and so the geology and environment are well-understood. Pacheco’s advantage is just about San Jose: it offers it a faster connection to Los Angeles, and also the prestige of being on the main line rather than on a spur that would have gotten canceled as soon as costs ran over.
The fact that Altamont is no worse than Pacheco at connecting Los Angeles to San Francisco, as opposed to San Jose, is the key here. Altamont has other advantages, but since the biggest advantage of triangles here is reduced to connecting a secondary city better, there’s every reason to prefer the Y.
The same is not true elsewhere. Let us consider three cases: New York and New England, Texas, and the eastern part of the Midwest.
In the Midwest, this is the easiest. The question is how to connect Chicago to Detroit, the options being the I-94 corridor through Michigan, and the I-90 corridor through Indiana and Toledo, which would be shared with a connection to Cleveland. In this case the savings due to picking a Y rather than a triangle are much greater, while, again, the Y does not compromise Chicago-Detroit, but only reduces Chicago’s connectivity to small cities on I-94 in Michigan. Unsurprisingly, there is no longer a debate I am aware of; the SNCF proposal and the Siemens proposal both connect Detroit to Chicago via Toledo.
In the other regions, it is harder. When one leg of the triangle is obviously more important than the other two, it can be useful to have a T, which is like a Y except that one leg is straight and the other two are lengthened slightly more. If Houston and San Antonio swapped locations, it would be obvious that it should be a T. But given that they are where they are, the strongest leg, Dallas-Houston, has nothing significant in between, while Dallas-San Antonio has two intermediate cities in addition to Austin, complicating that kind of T. The Texas T-Bone alignment keeps straight Dallas-San Antonio, the second strongest leg; on this rudimentary list of possible alignments on Keep Houston Houston, a T with Dallas-Houston straight does not even appear. SNCF’s proposal starts with Dallas-San Antonio and is agnostic on whether to extend to Houston as a triangle or a T.
Practically any solution but a triangle would make the weakest leg, Houston-San Antonio, more circuitous, but various compromises that keep it at least competitive are incompatible with making both Dallas-San Antonio and Dallas-Houston straight. The presence of Austin also makes an exact triangle infeasible. Houston-San Antonio on I-10 is 321 km; via Austin, it is 389; via the T-Bone, it is 500; via Dallas, it is over 800, making it completely uncompetitive with driving. The Interstates had an easier time – cars can get from Houston to Austin, Temple-Killeen, and Waco on state roads, and because 1960s’ Texas was empty between the three Triangle cities, construction costs were low.
In the Northeast, there is also an opportunity for a triangle versus Y argument, in the New York-Boston-Albany triangle, but this time the Y is weaker. The problem is that New York-Boston is by far the strongest leg and the first that should be constructed. For that leg alone, the advantage of a shore route through Providence over an inland route through Hartford and I-84 is not overwhelming, but it requires less construction (New Haven-Kingston vs. New Haven-Boston). On top of that, the pure Y would not use I-84 but require New York-Boston trains to go through Springfield, lengthening the trip, and even that would only make the extra construction required even with the triangle. On a high-value, relatively short corridor where every minute matters, this is a problem. The only leg that works either way, Boston-Albany, is by far the weakest.
Meanwhile, the second leg, New York-Albany, would greatly suffer from any such detour. New York-Albany direct is about 230 km. Via New Haven and Springfield, it’s 330, and the average speed is also lower because of unfixable curves between New York and New Haven and several forced station stops. On top of that, although less overall construction would be required at the end, New York-Albany direct requires less tunneling than going through the Berkshires, even with the Hudson Highlands, and also less urban construction through Hartford and Springfield. (Without the Y, New Haven-Hartford-Springfield would be an upgraded legacy corridor, rather than a dedicated HSR line, which would provide similar local functionality but be insufficient for an intercity through-route to Boston or Upstate New York.)
What this means is that just because a Y is preferable to a triangle in one location does not mean Ys are always better. It depends on how it impacts the stronger legs, on phasing, and on very dry constructibility questions. “The Altamont of X” is incomplete; the Altamont Y is special in that the strongest leg is indifferent to Altamont vs. Pacheco, making the benefits (as opposed to costs) a matter of 10 or 20 extra minutes on secondary markets.