SRNW signalling

openttd-srnw.org

@@ -143,3 +143,35 @@ unreachable way-point. A train can only have 255 orders. Once the
 order list is full no implicit orders can be created.
 
 [[file:openttd-srnw/pick-up-train-orders.png][file:openttd-srnw/pick-up-train-orders_small.png]]
+
+* Self Regulating Network Signalling
+The interesting bit happens once the trains passed the INJECT
+way-point. They are now inside of the waiting loop and try to get
+to the transfer station. This is only possible through one of the
+SBahn stations.
+
+When a train reaches the tunnel entrance to an SBahn station it has
+two choices, either continue in the waiting loop or enter the station.
+
+If a train is already in the waiting bay of that station we want our
+train in the waiting loop to continue and try the next station. We put
+two-way block signals at the tunnel entrance which turns red when the
+waiting bay is occupied by a train. OpenTTD's path finder treats a red
+two-way signal as a dead-end[fn::This is only true if
+=yapf.rail_firstred_twoway_eol= is =True=. This is the default in
+OpenTTD 13.]. It has an infinite penalty and any other
+path will be better. Our train continues. This is the first block
+signal in the picture below.
+
+At our second SBahn entrance the waiting bay is free and the block
+signal shows green, so a train should enter the waiting bay towards
+the station. However, the path finder sees that the path goes through
+a station. Going through a station incurs a penalty for that path and
+it might so happen that the path continuing in the waiting loop is
+considered better, our train would not enter the waiting bay.
+
+We need to add a penalty to the waiting loop path to make the path
+through the station look better. We do this by adding a few reversed
+path signals after the choice.
+
+[[file:openttd-srnw/srnw-signalling.png][file:openttd-srnw/srnw-signalling_small.png]]