Good point. PTC is designed to prevent specific types of occurrences. Amtrak’s PTC is pretty lightweight compared to systems like I-ETMS and does basically three things: enforcing civil speeds, enforcing positive stop, and enforcing temporary speed restrictions. The other requirements of PTC systems (such as separation of trains going in the same direction and protecting mainline switches) are met by the existing signal system and ATC system. Only when both PTC and ATC are functional and cut in is there a “complete PTC system.”
Some of the statements made above suggest a certain lack of understanding of what PTC is.
PTC is a set of specifications spelled out in the relevant CFR clauses, to be met to be certified as PTC compliant. It is required by law that mainline tracks meeting certain threshold criteria and trains operating on them be PTC compliant by 31 Dec 2020.
PTC can be implemented using many technologies. I-ETMS is one of them. I-ETMS implementation comes in many profiles. Some are overlays on existing track circuit based signaling systems and others are pure GPS based systems.
Another implementation of PTC is in the form of the ACSES II overlay on the classic coded track circuit based cab signaling system with signal speed enforcement (CTCCS - Coded Track Circuit Cab Signal). ACSES also comes in multiple different implementations (NJT ASES, MNRR ACSES, LIRR ACSES) which may have different speed codes in the underlying CTCCS. This is the most common implementation used in the Northeast on predominantly passenger lines. The original PRR system was enhanced with additional aspects using a second carrier frequency to achieve additional signal speed aspects above 80mph.
It is inaccurate to say that I-ETMS is superior to ACSES+CTCCS system. Indeed in at least one way the latter is superior as it handles train integrity and location of the rear end of the train better than a pure GPS based I-ETMS system, which has relatively vague idea about where the tail of the train is. ACSES also handles precise track location and higher speed operations more reliably than I-ETMS, specially above the speed of 100mph. I-ETMS is better at handling trains with widely varying operating parameters as one is wont to find in the US freight railroads. ACSES is better at handling standard train consist with relatively narrow bands of braking characteristics as is found in most passenger systems.
Incidentally there are track segments that exist where both ACSES+CTCCS and I-ETMS are installed. They interface with each other in the back office to provide a consistent view to ACSES or I-ETMS equipped trains operating on that segment.
There are locomotives and cab cars that are equipped with both I-ETMS and ACSES (and possibly additional PTC implementations like ITCS (Amtrak Michigan Line), eATC (FECR/Brightline, Utah Front Runner) etc.) too so that they can operate on tracks equipped with different systems seamlessly.
The biggest challenge that is being faced by the railroads at present is completing cross testing of equipment on different host systems. Since that presents an n x m kind of complexity it is potentially quite time consuming to get all the cross operations certified even after all the tracks are duly equipped with the PTC implementation of the railroad's choice.
