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Permissive signals - what are they for?

Started by merry, July 10, 2020, 04:07:25 PM

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I've been playing the current 'stable release' of experimental for some time, and on the whole it's a good simulation. But I have a few possible bugs (to report separately), and one feature that doesn't seem to have any effect: permissive signals.

Now, I understand Uk signalling principles in moderate detail. No worries with AB, TCB and token working of various types. I can make a stable and reliable signalled network (provided the block error gremilins stay away - see bugs...). But I can't for the life of me see how 'permissive' signals actually do anything! And I don't see any documentatoin (including the videos) on permissive signals either.

If I place a permissive signal (say on a loop - either at the entrance or further along - or at the entrance to a platform) it jsut acts as a standarrd stop signal. The trains, whether freight or passenger, do not stop and proceed at caution, which is the classic UK definition of permissive block as used on goods lines across the land.
So, what are they for? How shoudl I use them? Or are they just clutter on the menus?!


Permissive signals work only if there is no junction ahead of them, all the way up to the next signal.
Then trains can pass them on danger, and continue in drive by sight up to the next signal.

So there is no sense to put them in passing loop - if they worked in such case, it would lead to deadlock. Their use is on double track, preferably with short blocks, so the distance travelled in drive by sight is not long.


It can be useful to improve the capacity of platforms, so a train can start entering it as soon as the previos train starts departing.


It's best to use them on busy lines, where it's important to have your trains running even at a slow speed rather than being stopped.


Quote from: Freahk on July 10, 2020, 09:40:06 PMsy lines, where it's important to have your trains runn

From memory the Thameslink Core uses a form of permissive signalling along the length of of platforms, Farringdon being a good example. I'm not well versed on London Underground signalling but I think it's used there too, often to keep the high intensity service moving around station approaches.

They're also used on split/joining which isn't simulated as you know.


I'm well aware of the varous forms of permissive signalling applied in UK practice - traditionally, goods lines with permissive block allow multiple goods trains to queue in an AB block section; both traditionally and now, call-on signals at platform entrances allow both shunt moves and permit a second train to enter a part-occupied platform. Of course, modern transmission-based systems take signalling to another level and are more complex but let's ignore those for now!
The issue has been that, for me at least, permissive signals have simply not worked permissively in Simutrans for me, acting only as normal signals even if on plain line and with freight trains only.
However, having manually updated the package recently (i can't get the Nightly updater working!), I shall give it another go, in case fixes have helped.

BTW Thameslink core doesn't use permissive signalling, it's transmission based signalling with (can't remember) shorter or moving blocks. I am told that LUL doesn't generally use permissive signalling but instead has long used things like overlapping blocks and closing-up signals to allow trains to run safely close together (and of course the transmission based systems on the Jubilee and Central, amongst others).


Merry - I am not aware of any outstanding bug reports relating to permissive signals. If you find that these are not working, I should be grateful if you could post a bug report in the development board together with a reliable reproduction case. Thank you.
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