lensovet wrote:it would be like having an ABS system that's the same on all cars and doesn't take into account how fast the car is going or its weight or anything and saying "oh, ABS sucks! of course people can pump the brake better than this stupid machine!"
Also - this occurred to me as I gave it a bit more thought. ABS is not designed to guarantee a stop at a definite point in all conditions. It's a safety system that ensures that brakes don't lock up in a "panic stop" situation, at its core. It doesn't "think" or "analyze", per se. It simply helps the human operator not to brake his vehicle into an uncontrollable skid. Built into what it "knows" is the make and model of the car, its weight, and its basic performance characteristics. But that's about it.
PTC, on the other hand, has to be able to bring a train to a dead stop at a very specific point (or at least within some manageable, acceptable error band). It has to be calculating, on a second by second basis, the position, trajectory, and speed of the train and to be comparing that against what's in its physical characteristics database (which will provide permanent speed restriction locations, lengths, and speeds), its database of temporary speed restrictions (for temporary speed restrictions, lengths, and speeds...plus 'on the fly' restrictions that come up during the day as MoW requests track and time, foul time, etc.), and the states of all of the signal appliances in the vicinity (signal aspects, switch positions, and so forth). It has to do this for a trainset that might have different performance characteristics from day to day (especially in the case of freight trains), different engineers, and a different set of track conditions - dry, wet, high adhesion, low adhesion. There are certain things that the PTC system can "know" with accuracy, and others that it only "knows" to some lesser degree of accuracy. As Donald Rumsfeld once said, "There are known knowns;....known unknowns;.....[and] unknown unknowns." It's those known and unknown unknowns that force on-board PTC systems to add in "fudge factor" to their second by second stopping and reducing distance analysis.
This is why ABS is not an apt comparison, and also why there is some amount of safety margin built into PTC's calculations...which causes PTC, deployed as a simple safety overlay to existing train control systems, to be a drain on capacity.