Sorry to break up the impromptu eastoid convention in the midst of a Metrolink thread (
), but would the new PTC system be at all compatible with the ATS now used for high(er) speed running south of L.A., or is that just a vestigial Santa Fe remnant that would be discarded in favor of entirely new technology?
In its simplest form an ATS consists of an indiuctor mounted by the track which is activated when the signal is at danger and which is "read" by the locomotive to activate the brakes.
A PTC system is typically overlayed on the existing signaling system. However, it would most likely render the specific ATS stopping function redundant. Read on if you want a little more background on why and how....
In a very simplified description a PTC system needs to know where the train is located and that it is still in one piece and the speed at which it is running. Of these, in a track circuit based system you get a crude integrity information from the track circuit, and the speed information from the speedometer. Normally the only location info you have is very crude i.e. the train is occupying a track circuit block. The train can compute more specific location info by knowing the location of signals or other marked spots on the track that it just passed and adding to it the distance traveled since then. Then given info about next signal aspect and position of next signal and hence distance to next signal, a brake curve appropriate for the train type can be applied to bring the train to a stop before passing the next signal if the signal is at danger, or speed modulated to meet whatever the signal aspect says.
Now The old Santa Fe ATS I believe is based on a signaling system which is based on track circuit and a very simple stop inductor placed associated with each signals to enforce stop. The loco basically senses the need to stop and applies full service brake (I believe - not fully familiar with that system). This works as long as trains don't travel too fast and signal inductors are placed appropriately to ensure a stop before any fouling happens.
A PTC system overlayed on such a system would typically use new technology to get integrity information. It may get location information from GPS, since the whole idea of the new PTC is to avoid placing anything on tracks. PTC systems are designed to use the signal system that is in place to get signal aspect information, i.e. they are an overlay on the existing signaling system. Signal information will be delivered to the train by radio. Given the current location, speed, next signal location and hence distance to it, and its aspect, the onboard computer will (a) display all this on the console with appropriate warnings and (b) absent appropriate action by the engineer, will enforce the necessary brake curve. So even if the ATS inductors remain in place, in a properly operating PTC situation they should never have to trigger a brake application.
The PTC installed and operational on the NEC and apparently to be installed on many connected Commuter services (MNRR, NJTransit, Springfield, Harrisburg, SEPTA, MARC Penn Line) is and will be based on enhanced coded track circuit cab signal (CTCCS) + ACSES Phase II. Enhanced CTCCS provides speed enforcement based on either the original 4 aspect or an enhanced 9 aspect cab signaling system. ACSES as an overlay provides positive train stop + enforcement of speed restrictions at a finer granularity that possible with the CTCCS, including TSRs (Temporary Speed Restrictions). This it does mostly through Transponders placed on tracks adjacent to the signals. ACSES also provides a radio link for handling TSRs and various other more esoteric functions, like granting permission to pass a failed signal at danger etc.
Incidentally, the system operating on the NEC is a close cousin of the TVM430 system that is used on the French high speed system. It is designed and delviered by the same Alstom in collaboration with Amtrak. Instead of using the more sophisticated coding used in TVM 430, an extended coding overlayed on existing PRR coding is used on the NEC.
So a head locomotive that operates on the NEC and say on CSX to Jacksonville, has to be equipped with the the appropriate antennas and on board equipment to handle ACSES and whatever hybrid system gets installed between Washington and Richmond and then furthermore whatever PTC CSX chooses to install south of there. There is some hope that at least all of the GPS based systems will be able to share a lot of equipment in common, just like all of the coded track circuit based equipment is able to share a lot of equipment in common. Incidentally Washington to Rochmond already has a coded track circuit based cab signaling system.
Now there is a specific technical issue that is still not fully resolved. The requirements of the lineside to train protocol for controlling heavy slow freight trains in relatively sparse traffic situations is quite different from that for controlling high speed passenger trains on dense traffic extremely short headway situations. For the former it is easier to use radio since time is not as much of essence there and communication latencies and occasional failures can easily be worked around. For the latter redundant communications and fall backs have to be designed carefully, or face frequent extremely sub optimal operation while the system recovers from the inevitable glitches. ERTMS 2 based on radio beacons is operational in many places in the world (outside the US) now for 200+mph operation, but that is a very different beast from what the American PTC is designed for. Hence the general comment that the same PTC system does not necessarily work for both slow freight and high speed optimally.
Well, that became quite a long answer to a short question, but as you can see, there are complexities and nuances to worry about regarding PTC, just like for any other safety and security system that is automated.
