The debate about what kind intercity rail to build tends to be either/or. On one side, there’s HSR-only advocacy: this represents the attitude of SNCF, especially in the earlier years of the TGV, and such American HSR proponents as John Mica. In this view, legacy rail is inherently slow and money-losing and the best that can be done is to start fresh; generally, this view also looks down on integration with legacy regional rail. On the other side, there’s a legacy-only advocacy, which represents how Britain upgraded its intercity rail network in and after the 1970s and also the attitude of proponents of Amtrak-plus lines in the US.
The problem with this is that there are a lot of different markets out there, and the service levels they justify and the construction challenges they impose are different. Sometimes such markets are in the same general area, and this means some lines should be HSR and some should be upgraded low-speed rail.
Countries that tried to go to one extreme of this debate are now learning the hard way that they need to do both. Britain radically optimized its intercity main lines, which now have the highest average speed in the world except for HSR – but it needs more, and this requires it to build a new HSR line at immense cost. In the other direction, France’s TGV-only strategy is slowly changing. SNCF still doesn’t care about legacy intercity lines, but the regions are investing in regional rail, and one region even uses the high-speed line for local service. Japan gets away with neglecting most of the intercity lines because its physical and political geography is such that markets that can support HSR dominate, but other countries cannot.
This means that best network design is going to have to deal with both approaches’ political difficulties at the same time. Upgrading legacy rail means upgrading legacy rail operating practices, against opposition of workers and managers who are used to old and inefficient ways of doing things. And building HSR on the thickest markets means giving special treatment to some regions with infrastructure that other regions don’t justify; it’s economically solid, but the optics of this are poor.
But the advantage of doing it this way from the start is that it’s more future-proof, and allows integrated design in terms of schedules, which lines are upgraded, how cities are connected, and so on.
Doing it piecemeal may require redoing a connection along a different alignment. The issue is that HSR compresses travel times along the line only. It’s like urban rapid transit this way, or for that matter like the air network. A legacy rail system (or a national highway system, or urban buses) has fairly consistent average speed. This means that in a combined system, the optimal path between two cities may not be the shortest path, in case one is close to the HSR trunks.
For example, look at Upstate New York. None of its four major metro areas is large enough to justify a high-speed connection to New York by itself, but all four combined do. Although international service to Toronto is overrated, it could be justifiable in light of Buffalo’s relative economic integration with Ontario and also the mostly straight, partially grade-separated right-of-way available in Canada; this would further thicken the market.
If we draw a rudimentary map of other desired connections, none thick enough to warrant more than an upgraded low-speed train, the fastest connections are not always obvious. For example, with average HSR speed of 240 km/h and legacy rail speed of 100 km/h, it’s faster to get from New York to Ithaca via Syracuse than directly via Binghamton. This is why the connection to Ithaca is through a line that points toward Syracuse, even if it’s not the shortest route to Binghamton. It’s one of many small local optimization problems.
More interestingly, we get a mini-hub in Syracuse. Although it’s the smallest of the four main Upstate cities, it lies at the junction of the trunk line and lines to Binghamton and Watertown, and also has secondary cities at the right location for regional rail. (The largest comparable secondary city near Rochester is Geneva, which happens to be close to and have a good rail connection to I-90, a prime candidate for HSR corridor; thus it should get commuter service using the trunk line, which would be far faster than an all-legacy train.) This means that schedules should be set up to coordinate transfers in Syracuse.
This is a normal way to set things up in an all-legacy format, as is done in Switzerland, but it can equally apply to HSR. The construction challenges on the Empire Corridor are nowhere near as complex as those in California, Pennsylvania, and other truly mountainous states, but they’re still nontrivial. But now that we know that Syracuse should be a hub, one answer to the question “How many design compromises to make to reduce costs?” is “Build just enough to allow integrated transfers in both New York and Syracuse.”
(In practice this means HSR arriving in Syracuse on the hour and in New York whenever convenient. The main intercity line into New York is the Northeast Corridor, a very thick market that at HSR speed would have enough traffic to support show-up-and-go frequency. This is not true of lines serving Syracuse; Watertown is not Washington and Binghamton is not Boston.)
The main cost of doing things this way is political. It requires willingness to both prioritize markets and cut construction costs, as necessary to build HSR, and improve legacy rail operating practices and carefully integrate services, as necessary to build a working legacy rail network. The fiscal cost is not outrageous – those legacy lines are cheap relative to everything else (rebuilding the unelectrified New York-Scranton line is $550 million), and HSR on thicker markets will at least partially pay for itself.
Once we discard the notion that present-day Amtrak operating patterns are adequate, the question stops being about whether one trusts Amtrak or not, and purely about how to build a new transportation network. And then the correct answer to “High-speed or legacy?” is “Both, seamlessly integrated with each other.”