Standard Map Whistle Signs
Add the whistle asset used in the map to your assets.def file, if the sign can be detected by the Activity system it will trigger the necessary processing
Invisible Whistle Sign (Addon)
Use the asset Xing Ahead (Invisible) kuid:39134:102210 and add the signs where you want the horn sounded. Note you will need to add the asset to your assets.def file. Do not modify the existing whistle sign (if any) and do not define the map whistle sign if using this method.
Whistle Definition in Assets.def
For either method to work you must define the whistle asset to be used in the assets.def file for your Activity. This is mandatory as simply adding it to the core would mean that all maps would have all signs (and cause unnecessary asset downloads) and more to the point if you used a map with a different asset the whistle sign would not be recognised. Refer to the Developer Dox for the format of adding a whistle definition.
In the case that there are no signs, add whatever a standard asset like Zec's ones on the new Shunt and put that into the assets.def.
The invisible is if there are already signs and they don't work which may be possible depending on the author. What the system does is look for assets that have the type of 'Whistle' and then scan for those and try to work out when to act on the 'Sound Horn'.
So the possible states...
The map has whistle signs and when added to the assets.def they work .. done
The map has whistle signs and they do not work use the invisible signs to add to the map so that the system can pickup when to act on horn sound
There are no whistle signs, add the appropriate sign and add it to assets.def
Note: The system allows multiple definitions of 'Whistle' so that the case of LH RH signs can be catered for or adding additional signs or whatever, as usual the system is as flexible as possible to cater for all of those 'butt' conditions that always pop up.