In my first rail-related post I defined three different terms: high-speed rail (HSR), medium-speed rail (MSR), and standard-speed rail (SSR). I've been reviewing the Federal Railroad Administration's rules about train speeds, and I now see that I need to add another definition.
There are two criteria that the FRA uses to set limits: quality of the track, and type of signaling system. For the track, there are 10 classes each of which has increasingly strict tolerance, maintenance, and inspection requirements. For signaling, there are three levels. Below 60mph (49mph for freight), trains can operate in "dark territory." There are no signals on such lines, and train movements are authorized either by the almost obsolete method of pre-written train orders, or by track warrants received by radio from a dispatcher. For trains to operate above 60mph, a railway line must have some kind of electronic trackside signaling. This is usually some form of colored indicator lights, though in the past it also included mechanical semaphores. The signals can operate automatically based on how they are wired, or they can be controlled from a central location, or both. Above 80mph, the rail line and the trains operating on it above 80mph must have some kind of system that will automatically protect a train (usually by stopping it) under certain conditions. Unfortunately, how the protection is implemented and under what circumstances varies quite a bit between protection systems, of which there are several in the United States alone. But they usually combine in-cab signaling and a way of applying the train brakes without assistance from the train operator. In-cab signaling is present because the status of a signal becomes harder to determine at higher speeds, and even a brief moment of distraction can cause an engineer to miss a signal. In most instances the cab signaling is a supplement to trackside signals. However, on dedicated TGV lines SNCF has dispensed with all external indicators, as there is no need to support legacy locomotives without cab signaling.
Back to my definitions. My SSR standard would require Class 4 track and trackside signaling. Both are what can be found on most major rail lines, like BNSF's TransCon. Amtrak currently operates on some Class 3 and even Class 2 track, which is really somewhat pointless. The average speed for a train running on low-quality track is well below what can be achieved in a car, which most people would opt for. Even if all the bright new plans for HSR networks come to naught, Amtrak should make an effort to upgrade all of its routes to SSR standards.
The next level I defined was MSR, which topped out at 110mph. This corresponds to FRA Class 6 track equipped with automatic train protection. And 110mph just happens to be the maximum speed of Amtrak's two intercity diesel locomotives, the F59PHI and the P42DC. Various bits of track maintained and operated by Amtrak meet the Class 6 standards, as does a very limited amount of track owned by various state rail authorities. In the immediate future, most passenger rail projects will be building to MSR standards.
Finally, I defined HSR as trains operating over 150mph. That means I left a big gap between 110 and 150 mph. Amtrak's operations on Northeast Corridor fall right into that gap. I'm going to call a new standard for that range of speed ISR, where "I" stands for intermediate, or improved, or impressive, or maybe something else that sounds catchier. This standard corresponds to Class 8 track, again with automatic train protection. The reason the ISR standard is set at 10mph less than FRA allows on Class 8 track is because the lower speed is what Amtrak has determined as safe for its equipment and track. "True" HSR rail lines would have to conform to Class 9 standards. No track of this quality has been built in the US to date.
One more thing about the ISR standard: it is - or should be - irrelevant. Away from the Northeast Corridor and the Keystone Corridor, there aren't many existing right-of-ways where triple- or quadruple-tracking is possible. If a passenger-only right-of-way is being built out, it should be done to HSR standards so as to get maximum benefits. For track shared with freight traffic, upgrading beyond the MSR standard is pointless because the much heavier freight trains will throw the track too far out of alignment for higher speeds to be safe. Curve radii are likely to be too small for high speed operation on existing lines, as most were laid out by the 1890s, when expected speeds were much lower.