Jtgshu wrote:From fellow engineers who run the PVL, they said overall speeds significantly increased (actual running times, not timetable or scheduling) after it was shut off, because the braking curve no longer had to be set for "worse case scenario" which quite often (never) was not the case, and the engineers only had to comply with the cab signal changes - cab signals no waysides was installed - Norac rule 562. And passenger, especially short commuter trains with EP brakes are considered racecars compared to a heavy freight train braking in the rain or bad rail conditions - aka worse case scenario....
Jt, you're hitting it right on the head from the 'real world' perspective of a passenger train engineer, as usual. Thanks for the input!
The problem with safety overlay PTC (which is what is being designed for US railroads) is that it must assume worst case braking, track condition, and engineer experience in enforcing speed restrictions.
It works like this. If you're at track speed, say 70 miles per hour, and approaching a 30 mile per hour speed restriction (maybe it's a curve, say), under current wayside only, wayside with cab, or cab signal only train control systems, the judgment call about where to begin braking for the speed restriction is usually left up to the engineer (with some exceptions, such as the Elizabeth S-curve on the NEC, but those exceptions are few and far between). You get, with an experienced engineer, a fairly optimal braking curve, where the train doesn't begin slowing until it pretty much has to, to be down to 30 mph just in advance of the curve.
Now, add PTC. PTC, of necessity, must assume that an inexperienced engineer is at the controls, that the rails are coated with a slick slime that reduces adhesion significantly, and that the brakes are not working at 100% efficiency when it calculates where braking needs to start to get the train down from 70 to 30. That's PTC's enforcement point, where the onboard system will take over and slow the train if the engineer fails to do so. Because you've assumed worst case conditions, you're already backing off from the "engineer's best judgment" braking point by a fairly significant distance. Next, you have to consider the PTC warning curve, which becomes active *before* the PTC enforcement curve. This is the point where the PTC system alarms to let the engineer know that he/she is about to get a forced brake application (AKA a "penalty application") if he/she doesn't initiate braking. This can be set from 10 to 20 seconds or so before the automatic system cuts in. Finally, you have "pre-action". This is the phenomenon, in human factors, wherein the engineer, once he or she has operated in the PTC territory several times, begins to 'know' where the alertor will sound, without even thinking much about it. He or she will begin to get on the brake a little early to avoid the annoying alarm. So, now we've not only backed off from the "engineer's judgment" optimal braking point some distance due to our assumption of worst case conditions; we back off further to allow for the warning alarm; then we back off further because the engineer's reaction to learning where the warning alarm occurs each time will be to begin braking before the alarm goes off.
The result? You slow the train down a surprisingly and distressingly long distance from the "optimal" point where a good engineer, using his/her best professional judgment, would start braking. Thus, you lose capacity. And don't get me started on the problem of absolute signals located at the leaving ends of platforms under PTC...