ATP "release speed"

[PJW]

Hi Signalling engineers:

Recently, I am reading article about the ATP system "An Introduction to Intermittent and Continuous ATP System" and in the content it has introduced the term "release speed" for ATP system, but I don't quite understand it.

I have looked through website, but has not yet found anything about it,so can anyone explain what is "release speed" in ATP system and does it have anything to do with the overlap for ATP ??

thanks

With an intermittent ATP system the train will get be told to stop and will not be able to start again becuase it does not get the update until it passes the signal.

The release speed allows the driver to obey the aspect and allow the ATP to get the required update. If the driver does this in error, a SPAD will be detected and the train brought to a stand, hence the release speed is dertermined by design of the conflict point. This could be seen as an ATP overlap (I think it gets called this in the ERTMS application).

In essence with any form of ATP system:
a) the driver is responsible for stopping at the Limit of Movement Authority which is at the signal (where provided) and the ATP supervises this.
b) However the ATP is responsible for stopping the train before it reaches the conflict (generally the end of the overlap); there indeed needs to be a margin so that the driver is free to drive their train without the chance of unwarranted interventions. Therefore the ATP does not worry about the need to stop the train prior to the signal but waits until it is certain that driver has made error before applying the emergency brakes.
c) The ATP supervises the drivers braking against the service braking curve (actually there are often 3 targetted at various stopping positions just prior to the signal- an annunciation curve to give a warning, the curve the driver should really be following to be able to stop a defensive driving distance prior to the signal and the extremely last moment curve used to intervene if driver hasn't reacted). Once the speed has been reduced the defined sufficiently low level known as the "release speed", it no longer supervises to this cuurve but permits the driver to proceed towards the signal at this (or lower) speed. The primary reason why is that the ATP never quite knows how far there is left to go due to possible odometry error. It is a bit like feeling ones way cautiously in the dark; when you think you are getting close, proceed very slowly but be ready to react very promptly!). The ATP will find out exactly where it is once the driver SPADs and although it is obviously too late to stop at the signal, provided the speed is low enough then safety is not compromised. The ATP permits the SPAD to occur at a speed up to the "release speed" because is knows that it will be able to stop the train prior to the possible conflict. The driver will be in a lot of trouble of course, but no collision will result.

Hence the calculation of the release speed certainly depends upon the overlap length but also things such as gradient, odometry and possible lurch uncertainty, latency in performing calculations, the brake build-up time, emergency brake rate, any extra contingency margin etc.

The longer the overlap length, then the longer the "release speed" could be. Not necessarily a good idea though. On the Cambrian ETCS (OK this is a level 2 system without lineside signals rather than strictly an ATP as a supplement to convenntional lineside signalling) there have been some post commissioning issues re driver confusion. Basically, due to the passing loop geometry, there are some short overlaps appropriate for class 158 DMU passenger trains and some separate much longer overlaps for freight trains (have much worse emergency braking due to brake build up time and their weight). If a signaller selects the long overlap for a DMU, then the driver's in-cab display shows the driver that they indeed need to stop in a certain distance but also displays the release speed- given the long ength available compared to the DMU's braking distance then this can be as high as the permissible speed of the line, I understand. Hence can be prone to mis-interpretation by drivers not very familiar with the new system; they have been known to continue past the place they should have stopped and then finds the ETCS intervenes and that a SPAD has been committed.

In an intermittent ATP system then the same applies, but with another factor to consider. Assuming there is only transmission of information to the train in the immediate vicinity of each signa,l then it may be that the driver can see a green signal ahead but the ATP system doesn't yet know that it has cleared. If the train had passed the previous signal at yellow, then the information that the following signal was red would be being stored by the on-board. The ATP must continue to assume that this is still the case, until it gets an update and for that it has to wait until the train encounters the next loop / beacon. In the extreme case, there will only be data transfer just beyond each signal and thus the driver must be permitted to continue, albeit at a low speed, in order to get to the next beacon or there would be an impasse. [In many cases however, note that even an intermittent ATP system has an extended area of transmission that might extend a few hundred metres on the approach to all / many signals]