Signals question

z-z-z · Post by **z-z-z** » 24 Jun 2007 15:34

I'm not sure it's the right forum section for such questions, but anyway, here goes..

So, I have two trains, first going from A1 to A2, second -- from B1 to B2, see attachment. Now I want to add another track C to this layout, which will be used by a train going from, say, C1 to A1.

Obviously signals must be placed on intersections of C and A, and C and B to protect the crossing and a station at A1. However, when train goes to AxC (top crossing), it also blocks the bottom crossing, which is not in any way related to it, at least not for my purposes.

Can anything be done about it, short of making C not cross B at all?

kaan · Post by **kaan** » 24 Jun 2007 15:43

Try making room for a signal in the middle like in this screenshot.

z-z-z · Post by **z-z-z** » 24 Jun 2007 15:55

Well, that's an option, but then there is a station at A1.

So, if I put a signal in the middle, then train going from C to A1, will stop before that signal and right at the bottom crossing, which is going to be even worse, because it will block B for a longer time.

z-z-z · Post by **z-z-z** » 24 Jun 2007 15:58

Actually, I can use pre-signals for the middle signal and the one before CxB. Thanks.

cmoiromain · Post by **cmoiromain** » 24 Jun 2007 21:10

or even better: use a bridge or a tunnel. then you wont have any problems, they won't cross at all

z-z-z · Post by **z-z-z** » 24 Jun 2007 21:31

OK, that idea with a signal in the middle worked for a while, however Bloggs Express now wants me to add another line to this junction.

I'm using pre-signals to communicate station status, but it seems like if I put another signal in the middle of junctions all this scheme stops working. And without it, trains coming from A1 to A2 block crossing at B and vice-versa.

To be honest, this isn't much of a problem, but it would be very interesting to find a solution. Mostly because these two track (A and B) are really unrealated, they just happen to cross.

cmoiromain · Post by **cmoiromain** » 24 Jun 2007 21:38

put a two way combo signal on those straight pieces of track between line A and the crossings with line B

then a train passing on line A wouldn't be on the same piece of track as a train on line B (with no signals in the middle, it is considered as being on the same)

z-z-z · Post by **z-z-z** » 24 Jun 2007 21:46

cmoiromain wrote:put a two way combo signal on those straight pieces of track between line A and the crossings with line B

This doesn't seem to work. Looks like combo signals do not pass info to each other or something, but in either case, if I do that, then entry signals show green lights even when the station at A1 is full.

cmoiromain · Post by **cmoiromain** » 24 Jun 2007 21:52

then there is a bug... normally it should work...

Willsims · Post by **Willsims** » 25 Jun 2007 01:25

Your pre-signal on track A is too close to the junction, and anything but the shortest train will block the junction if it is brought to a stand by the signal. The simplest way of solving the problem, to my mind would be to use a simple 2 way signal on line A, one at each side of the junction, and a 1 or 2 way signal on lines C, depending upon their configuration. If you want to use a pre-signal to protect the station, the section between the normal signal at the junction, and the pre-signal must be long enough to hold your longest train. This will not entirely prevent the junction being blocked as if a train is stood at the pre-signal, a train will still enter the junction, but will not be able to exit it. I'm afraid I don't know that much about pre-signals, but if they work in combo with each other, then pre-signals protecting the junction may work. If you have a 2 platform station, with no crossing at the throat, do you really need a pre-signal in front of it; won't the train automatically chose the empty platform anyway?
I would definately use a bridge or tunnel to cross line B, as that is really an irrelevence to the problem; and if you are constructing an underpass beneath line B, you might as well continue it under line A, and form a grade seperated interchange anyway, which should solve the problem entirely.

z-z-z · Post by **z-z-z** » 25 Jun 2007 02:24

Willsims wrote:I'm afraid I don't know that much about pre-signals, but
if they work in combo with each other, then pre-signals protecting the
junction may work.

I think the problem here is that combo pre-signal is not combining well
if I put another 2 combo pre-signals in the middle between the C-B and
C-A junctions.

Willsims wrote:If you have a 2 platform station, with no crossing at the
throat, do you really need a pre-signal in front of it; won't the train
automatically chose the empty platform anyway?

Sorry, it's not visible on these screenshots, but this station has no other
exits, so trains must leave the way they came in.

So pre-signals are needed keep trains from A2 and C out of the junction
until station has empty space and make them wait in front of the crossings,
not on them. For example when train comes from C it stands before the
entry pre-signal and the crossing can be used by trains moving along route
B, and trains from the station can leave it without any problems (almost
always, but that's another story).

Willsims wrote:I would definately use a bridge or tunnel to cross line B,
as that is really an irrelevence to the problem; and if you are constructing
an underpass beneath line B, you might as well continue it under line A,
and form a grade seperated interchange anyway, which should solve the
problem entirely.

Well, I guess, the easiest approach would be to just demolish some
buildings and separate all three (or at least two) of the tracks. It's just that
I don't find this approach to be sufficiently interesting.

Willsims · Post by **Willsims** » 25 Jun 2007 03:07

If there is no other exit from the station, then any train standing at the pre-signal on line A will block the track, whether it is on the junction or not.
If I'm ever buiding a multitrack station, to be used by more than one train, I always have a good length of double track approaching the station with one way signals, a crossover about 1 tile from the station itself. and a short length of bi-directionally signalled track between the crossover and the station. The track can be singled, with passing loops, further down the line, but trains are then well away from the Station and cause little congestion.
I very rarely use single track unless there is definately only one loco on it, and after the first few years, when finances are no problem, any line used by more than one train will be double tracked, unless there is an awkward town council which won't let me.

SoLo · Post by **SoLo** » 25 Jun 2007 05:39

With your 3 train track, did the train going to C ever get lost?

I'm pretty sure this track combination never actually worked fully. And if the train going to C never got lost and ended up going to A1, then you just got lucky.

If I am wrong though, the simplest solution that I see if to join tracks C and D (I guess it is D) before the junction. That way the lead into the junction will essentially be the exact same as it was before, with just one cross over track. And add signals a bit earlier to lines C and D to keep them from trying to get into the junction at the same time.

z-z-z · Post by **z-z-z** » 25 Jun 2007 07:05

SoLo
No, trains from C never get lost, yes -- it works fully, and yes trains from C
indeed go to A1, because that's where they are supposed to go. :- ) This is
described above.

As for joining C and D, this can help, yes.

To summarize there are currently two problems with this junction:

First, trains passing from A1 to A2 through top crossing block the bottom
crossing for trains passing from B1 to B2 and vice-versa.

This can be resolved by merging both C tracks and adding another combo
pre-signal.
Second issue is that sometimes, when there is an empty space at the
station the train leaving it can bump into an incoming train at the signal.
Thankfully this is very unlikely to occur, and even when it does a train
at the station's branch will back off and the delay will be minimal.

This can be resolved by removing the only combo pre-signal, but this is
absolutely undesireable.

I guess, I'm being overly verbose and excited, but you have to
understand, I played TTD maybe eight years ago last time and I'm very
happy about discovering OTTD.

SoLo · Post by **SoLo** » 25 Jun 2007 13:06

z-z-z wrote:SoLo
[*]Second issue is that sometimes, when there is an empty space at the
station the train leaving it can bump into an incoming train at the signal.
Thankfully this is very unlikely to occur, and even when it does a train
at the station's branch will back off and the delay will be minimal.

Yeah, that's the same problem I ran into. The A1-A2 line always had a green signal, even when both of the station slots were filled.

Maybe the train to C only got lost after I added the 4th train, might have confused myself, spent about 30 minutes playing with the 3 train and 4 train setup and may have mixed them up. Sorry about that. I also used a really short track (for a quick test), so the trains only took 4-5 days to get from A1-A2, and B1-B2, putting A LOT of trips through the junction, yours is A LOT more spread out so the junction wouldn't be filled as often.

I went through it again and the only way that I could find to get it to work (still has a chance of A jamming) is if C and D merge before the junction. It seems that if there is combo signal in the middle, the A line looks at it as a possible exit, and that is why it has the chance of jamming in the junction. So when you add a second line through, now there are two signals in the middle. So C sees the combo on D and counts that as an open path and D sees the combo on C and counts that as an open path, making it so that the entry signals are always green since atleast one of the middle signals is always showing green.

z-z-z · Post by **z-z-z** » 26 Jun 2007 18:54

Well, what can I say.

Eventually, I had to rebuild part of the junction using bridges because of
completely unrelated problem on the line B. I guess, I just was putting too
much faith into signals.

ostlandr · Post by **ostlandr** » 27 Jun 2007 01:50

Here's a suggestion of mine. I find that the presignal blocks at my standard stations are a perfect place to add in a branch line.

Warning: These can cause backups and gridlock on high-traffic networks as they only allow one train in the presignal block at a time, and trains need room to stack up while they wait. (They work fine on my low-volume networks, though.)

I also 'cheat' sometimes, and make one platform at the station exclusive to the branch line- this reduces congestion and keeps the branch line trains from wandering onto the main and getting lost. I had one stupid 55 mph wood train that kept wandering out onto the 80-90 mph main line and slowing down my passenger trains and time freights. I ended up isolating the branch (two forests and a sawmill) from the rest of the line.
I also do that when I have an electric interurban "trolley" line running into a city- I add a separate platform at the main station for the trolleys.

Vloris · Post by **Vloris** » 27 Jun 2007 09:35

ostlandr wrote:Here's a suggestion of mine. I find that the presignal blocks at my standard stations are a perfect place to add in a branch line.

Yeah, but that is really where the trouble starts! I sometimes have a train wandering of a branch like that! That's even more anoying than a lost train on the mainline, as the branch wasn't made for more than the two trains that normally run on it

In my experience such things happen much less when the branch line has it's own connecting junction, and my latest invention: make at least the connection to te mainline double tracked, so there are no double-sided signals there. That way, you cannot have a train from the branchline, and a train to the branchline meet head-to-head on one signal.

jwa1992 · Post by **jwa1992** » 27 Jun 2007 12:20

in my opinion this is the best way

Tekky · Post by **Tekky** » 27 Jun 2007 17:51

z-z-z wrote: I guess, I just was putting too much faith into signals.

As far as I can tell, there is no ideal solution using the current OpenTTD signalling system. All the proposed solutions in this thread will cause a gridlock sooner or later (of which some may be unlockable by automatic train reversal).

In reality, however, there is a possible solution to your problem. Your setup would look like the screenshot in my attachment. This layout will never cause a gridlock, in reality. However, this realistic layout will currently not work in OpenTTD, for the following reasons:

OpenTTD consideres track with single signals as one-way track.
OpenTTD requires the placement of exit signals after intersections to mark the end of the intersection block, despite trains never being supposed to wait in front of these exit signals (since that would cause the train to block the intersection). In reality, signals only exist in places where it is safe for trains to wait (which makes a lot more sense). Therefore, in reality, the end of an intersection is considered implicit after every switch and exit signals are not necessary.
Trains in OpenTTD do not reserve a route to the next safe waiting location, e.g. to the next safe signal (note: the orange signals in the screenshot are considered unsafe waiting locations because they are on bi-directional single track) or station platform. Instead, they only try to move from block to block, which will causes frequent gridlocks when signals are placed on single bi-directional track.

For these reasons, I proposed a completely new signalling system in which signals behave as in reality, so that all realistic track layouts (such as bi-directional double track and bi-directional pass-through stations) would work.

This proposal also uses realistic path based signalling, which allows several trains to occupy the same block at the same time, provided that their paths do not cross.

Please see this thread for further information.

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