A bottleneck
An intersection is where a mainline crosses or joins with another
main- or branchline.
The intersection below will work well for a mainline that is not too busy.
If there is heavy traffic however, trains from mainline NW will interfere
with trains from mainline SE. It is unfortunate that they share a section,
the bottleneck, eventhough they do not cross eachothers tracks!
Of course this could be solved by using
a tunnel or a bridge
for the mainline SE to branchline transfer. But as stated in the
Lost trains and Signals -section
this might cause trains to get lost.
Here is a simple solution which doesn't need trains negotiating gradients!
Note the extra tile between parallel lines - space for a signal.
This way trains from mainline SE to/from mainline NW or mainline SE to/from
the branchline do not interfere with eachother. This results in less
(unneccessary!) disturbance of individual trains and therefore
higher revenues and better council ratings.
. - . - . - . - . -
Wycliffe, TV detective; BBC TV series, Cornwall .
Chief Superintendant Charles Wycliffe; Chief Superintendent.
1 .
Wycliffe played by theatre actor Jack Shepherd (I), director, writer of plays.
play, film, directed by, Jack Shepherd, theatre, actor, author.
b .
Now we can add the possibility for trains from the branchline to the
mainline NW.
In this picture you see that a deadlock can be caused by as little as
3 trains.
The lower the number the higher the risk that this happens.
Again, if traffic isn't too heavy things should work out.
But if there are many trains like 1 and 2 then the performance of the
junction will significantly deteriorate. Eventually the lockup will be
resolved when one of the trains reverses.
This will happen automatically after some time. But in the meantime all
trains behind the culprits will queue up waiting to continue their journey.
In fact if to the run up to the signals where all 3 trains are waiting
the track is covered with signals than no train can reverse and an
indefinate lockup has been created!
. - . - . - . - . - . -
Wycliffe, TV-series, TV detective, Cornwall .
Chief Superintendent Charles Wycliffe; Chief Superintendant.
2 .
Wycliffe played by theatre actor Jack Shepherd, director, author of plays.
play, film, directed by, actor, Jack Shepherd (I), theatre, writer.
s .
The rest of this chapter is from me, Cecilie Irgens:
Depot force - Depot pass
Use depots at tricky junctions! Often (but not always) 2 meeting trains
can solve their problem:
The popular triangle depot-force: Normally the signal on the main track
is one-way (TTd only); forcing the (empty ?) train going 'back' into the
depot.
If the signal on the main track is changed to 2-way, this functions as
a simple passing system:
Train A - the 1st train to arrive - 'sees' the other out there;
hurries into depot.
Train B uses the main track. Then Train A goes out, and gets a 'free'
depot visit on the way.
Quite useful when you can't afford 2 tracks all the way, or there is no room
for a long enough passing loop.
Are TTd's meeting trains more sensible than TTo's?
My experience so far tells me NO!
They seem even more 'impatient' than in TTo! zipping around immediately
to find another route
usually ... straight AWAY from the right one.
Next TT version: I should like "Train Patience" as a parameter!
Signal problems - pretend it's a horse!
Sometimes a very simple thing will fix a silly problem:
Train 31 from depot should follow the 1-way signals out to the right ...
ONE signal in the left pic makes Train 31 go wrong, back into station -
that red signal frightens ...
TWO consecutive signals in the right pic makes Train 31 go right,
follow the other (outside picture) train!
TWO 1-way signals in tight sequence - useful when you want the train to go
THERE and wait, rather than use an alternative route.
In my (Cecilie's) experience, the best way to "understand" the behaviour of TTd trains at signals, is to compare them with nervous jumpy horses!
My friend the Horse does not see very well - his No 1 sense organ is his large sensitive nose. Engines, trains, bulldozers ... they smell bad, and horses tend to swivel around and gallop away from them.
Our TT engine drivers' eyesight is selective: he needs to get real close to see a signal; but a TRAIN behind a signal - he smells that quite well! In fact, he sees a red signal with a train right behind it as more dangerous than a red signal with some empty tracks behind.
Look at our TT engine driver in the following picture: He skittered
away from the 2-way signal at B - there's a train behind it! ...
so he put his nose on the other track, at A - so short-sighted that he
doesn't, or won't - see that the A signal (red too!) is 1-way
against him!
Then he had no option but to turn and gallop off towards C. If the signal
there is 2-way, he keeps galloping ...
NB! The distance between the branch at A-B and the 1-way C signal must be
long enough for 1 full length train! Else the train will be stuck, unable to
right itself. You may have other signals inbetween, but only 2-way ones!
80% to 95% of the time, you can avoid the situation in the previous picture
if the signal at B is 1-way, leading into the depot.
You won't get 100%, since there's always a random element in a train's route choice.
The next picture - recent experience, 2001 - indicates that maybe
trains are programmed to think:
"If both signals are RED, always go for the 1-way - check later if
it's against me!"
. - . - . - . - . -
Wycliffe, TV detective; BBC TV-series, Cornwall .
Chief Superintendent Charles Wycliffe; Chief Superintendant.
3 .
Wycliffe played by theatre actor Jack Shepherd, director, author of plays.
film, play, directed by, Jack Shepherd (I), actor, theatre, writer.
b .
signals at depots
This picture is from an overcrowded complex junction, where depots give trains
more space for their waiting time. Thanks to the depot, there should be room
for train 4 to wait behind train 3.
I've used a 1-way signal on the IN side 'a', to stop train 3 from going out
the wrong way.
But observe! When both signals are RED, train 3 tries to come out
to the 'a' 1-way signal, although it's 1-way against the train!
Train 3 will turn back into the depot, then pop it's head out again and
again, always at the 1-way signal! until the 2-way turns green.
So maybe 'b' the OUT signal should be 1-way? I've tried that, but then the
trains never enter the depot, and this branch has room for 1 train only, not
2 as planned.
Why? I assume the trains are programmed to try any 1-way signal first,
then check .... I've seen this sort of thing many places.
For best action, you maybe need room for double signals both before and
after the depot:
- a 1-way followed by a 2-way before the depot;
- a 2-way followed by a 1-way after the depot.
So have I found the ULTIMATE 'train choose which signal' algorithm?
Not quite...
In this pic, my Train 27 ignores a perfectly green 1-way signal at 'a'
- there's another green signal 'b' behind it! Instead the Iron train chooses
the old SLOWer track, the 2-way signal! why?????
2 things may explain it:
1: Trains have a strong preference for the 'ordinary' rail - the one in
bridge/ tunnel direction - often the SLOWest route!
2: The ignored 1-way signal is on the same tile as that steel train,
although the 2 tracks are not connected at all.
Probably '2:' is the reason why, but I've seen examples of just '1:' too.
There's always a possible random element in a train's choice between signals.
Unfortunately.
-
Seabiscuit the racehorse.
Wycliffe TV-series BBC TV series TV detective.
Chief Superintendent Charles Wycliffe; Superintendant
4 .
b .
Other types of junction structure:
If you only copy other peoples' rail systems, you'll never invent anything
new. These 2 ideas are mine - I haven't seen them elsewhere.
As mentioned other places, it's often good to have 1 or 2 tiles of space
between your 2 parallel tracks. This allows connections between
the main tracks, combined with bridge / tunnel.
If your spaced tracks are at a level above the surrounding area, on
one or both sides, the "Slip-under" connection can be useful: The inside
branch goes down one level, then out via tunnel, or under a short bridge
connecting the main tracks.
This structure often saves pulling down an extra point, so it can save
some trees. WoodyVille will love you!
The TT signals are sometimes a bit too democratic - You don't want that full
high-pay coal train to wait for the 2-carriage passenger train ...
Here the short pink lines represent low priority trains; The strong RED lines
are high priority trains. (Some of those reds are enhanced.
By placing some of the signals a bit back from the intersection, one can
give the chosen direction priority.
Trains going S--N are considered important, and have priority over trains
from E or W. Train 'b' is nearer to the connecting point than train 'a',
but 'a' goes first - it passed the previous signal first.
Same principle for trains 'c' and 'd'.
My statement: "In a cloverleaf or other large junction, the branch-off
for turning left / right should be long enough to contain 1 max length train
between signals." Someone asked: "WHY?" Here's why:
In this picture train B waits for train C, which maybe waits for train D
(purple chalk line). This is normal, trains B and C are turning left.
But train A, headed straight West, also waits on the main EW track.
It shouldnt!
If the branchoff train B is sitting on had been long enough - B's tail clear
of the signal at 't' -
the track and signal in front of A would have been clear:
A could drive on. Other trains behind A too!
The main point of a junction like this one, using tunnel-under or bridge-over, is to get trains through quickly - no unnecessary waiting! For every train which has to wait on the main track, then slowly start up again, there's an increased risk of trains queuing up behind it, ... more trains starting slowly up, one of them having a breakdown ...
For a large junction, you should compute how many trains per month are likely to pass through totally, and in each direction. If more than 4..5 at each of the 4 exit points, you might easily have a bottleneck, even if the junction is well constructed.
NB: the easy turns - the turns right in this case - could well be moved out far from the main junction, giving the Turn-Right trains a shorter route! And removing these trains from the main junction.
NB: For any type of large junction to work well, ALL your trains should have a depot force once per round trip, outside the junction! If you forget this, your trains will go amok when 'depot time' comes around.
Roundabout junction with NS shortcut
If few of your trains turn left, and the traffic isn't too heavy, you
might - temporarily, maybe -
build a roundabout junction instead of a 2-level one. Advantages:
- Trains won't have to go uphill.
- The roundabout solution probably chops fewer trees. If near a town, you'll
be more popular.
My roundabout is provided with a straight-through shortcut for the most
important direction. There's 1 tile between parallel tracks, room for a signal.
The branches have depots, to provide longer rail-space for waiting trains.
Those signals (red lines) between Main Direction tracks:
Train 'b' is waiting for train 'a' and blocking train 'c'.
But at other times, the lack of such a signal would make train 'a' and 'c'
wait for each other - so which situation is most frequent?
The mainline 'a' and 'c' are supposed to be the most important trains.
There should be few of the turning type 'b' ones.
After the refuge depots: just ONE 2-way signal - makes trains enter depot
when that signal is red.
You can download
rounbout.zip,
a game-save with 12-16 trains with which you can test my junction.
Same trains as for the other junction, but don't expect more than 12 trains
to work well with this one. Even 12 trains is overloading it.
Later, when traffic gets too heavy, or many trains want to turn left, you probably need to rebuild to another type of junction, on 2 levels.
Cloverleaf junction, less pileup?
OK, so I built a large roomy righthanded cloverleaf junction based on the
principles I've stated further up:
- The 'easy' right turns are not implemented, or outside the picture.
- The left turns start with an early branchoff - see yellow lines -
to get turning trains out of the way of the main track ones.
- These branchoffs are long enough for 2 full length trains, + a 3rd one in
a depot just before connecting to the main track.
- Signals at these depots: see Signals at depots;
here I've placed double signals before the depots, hoping the train
inside will stay there until the signal after is green.
After the picture, I rebuilt to allow double signals both before and
after those troublesome depots. Result:
The junction works OK, but trains never visit my refuge depots!
Maybe further back, with 1 train length between signals AFTER the
refuge depot ... see my roundabout junction.
So: these depots aren't much use, really. Drop them if little space.
You can download clovrlif.zip, a game-save with 12-16 trains with which you can test my junction. Same trains as for the other junction. I've tested this one with 16 trains going around, it seems to work quite well.
"Short Left turn" junction
Turning is one of the things which slows down trains, and which might
result in a breakdown. So let's build a 2-level junction with "low curl" or
"Short Left turns".
I'd like a better name for it - "Cecilie Junction"?
This junction needs the parallel main tracks to have 2 free tiles inbetween.
The early branchoffs (yellow-green lines) go inwards, and pass other
tracks using a bridge.
Other parameters are as usual: The main direction stays at the same level.
the other one + the turning trains have to go tunneling / bridging.
Adding "Right turns" to this junction looks simple; you could connect them
to the existing refuge depots, shortly before connecting to the main tracks.
If your junction is near to a town with Tree problems, maybe you should first build a wide Roundabout, with connections at the right spots for rebuilding into a "Short Left" turn. The outside parts are about the same shape. this would chop the trees in 2 rounds, rather then all at once.
I haven't provided test trains for this one: Your job!!
But you'll find this junction, the "slip-under" construct and the priority
signal example near Vancouver, in
clovrlif.zip,
along with my cloverleaf version; and along with all the original stuff
created by James S. Baughn! I've built my extra structures onto his
"MicroSloth" game.
You might rebuild the cloverleaf to a "Short Left turn" junction, and then
test it with my 16 trains.
Last updated:
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,2003-0106,0102 ,(2002-0305,(0303,0225, (2001-1001))
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