Invisocar, the phantom AI sound car, version 2. 
Extract or move the invisocar folder to 
\Train Simulator\Trains\Trainset. 
If a previous version is installed answer yes to overwrites.

Coupled behind the engine(s) of an AI train, this car will blow a horn (diesel) or sound a whistle (steam) when first approaching the player train, within 900 meters and again at 500 meters distance from the player's viewpoint. Works in cab, passenger view, or externally. Zero meters long and invisible, it leaves no gap between couplers, so can be used for any AI unit train as well as mixed consists.

To use, add to an AI train consist with Conbuilder (current version is cb_160f.zip at www.train-sim.com file library), between the last engine or tender and the first car. Works with any engine without interfering with normal AI engine sounds.

I don't suggest using the activity editor consist editor because the texture is invisible, so the activity editor consist editor won't show it. In addition to that the wagon is zero meters long, so even if you add an actual texture it still don't show up, I tried it. It is just possible to add invisocar to a new consist, by adding the engine, then invisocar (dragging from the blank window!), then the rest of the train, but since it's impossible to tell where the car is you really can't edit it afterwords. If you have another program that displays cars as icons you could probably use that instead, but Conbuilder works for me.

WHAT'S NEW VERSION TWO:

Changed the length to 0.01 meters to fix problems when consists with Invisocar are driven as a player train. Changed coupling to rigid coupling to eliminate double slack action between cars. Added a bunch of new sounds, including one for Europe (aliases 380 whistle) and one for UK (aliases Scotsman whistle).

You can open any of the .sms files with wordpad and change the distances just by changing the "Distance_Dec_Past 500 PlayOneShot" number from 900/500 to something else. As far as I can tell these don't work beyond 950 meters, so changing to 1500 or something is not recommended. The "REV" sms files play 3 shots for each distance trigger instead of 2.

You can also change the horn or whistle sound, either by changing the name of the .wav file in the .sms file to the new .wav file, or rename/copy AIsteamwhistle.wav and/or AIdieselhorn.wav to a backup, then rename the new .wav file to match what the .sms file is calling for.

Harold Payne made two revised wags with the "REV" in them, these play 3 horn/whistle blasts per distance trigger instead of two.
 
There are three other new wags:

InvisocarSTM_Xing.wag
InvisocarDSL_Xing.wag
Invisocar_mp15ac_3Xing.wag

These are good templates for the grade crossing long-long-short-long signal that most people are familiar with, to make your own use something like Goldwave to cut the sound length in half, save as whatever_short.wav.
Open Diesel_Xing.sms in wordpad, replace the "../../GP38/Sound/x_gp_horn1.wav" (aliased to save space) with your regular horn (or whistle) wav, and replace "x_gp_horn_short.wav" with your new half length wav. Then save as a different name, open one of the .wag files, scroll to the bottom, change the .sms to the same .sms name you just made, save as whatever_you_wanna_callit.wag.
The two samples I provided can be used as is in a consist, they play at 700 meters. You can reduce that, but if you make it 100 meters the player will wonder why the AI is blowing for a grade crossing when there's no grade crossing in sight! 

If you make and upload an activity that uses this car in your AI trains, you could include invisocar in the zip. The whole thing zips to about 550k, so including it with your activity instead of instructing the activity player to download it shouldn't be a problem.  

Some sound files were renamed and some default wav files were deleted and aliased to the original engines, so this requires that the default 380, Scotsman, GP38, SD402, and Dash9 be installed for the aliased sounds to work.

There are now 14 invisocars including one for Europe (aliases 380 whistle) and one for UK (aliases Scotsman whistle).

Known issues:

When coupled between two back to back engines this will either derail or uncouple on tight S curves. Best way to avoid that bug is to couple behind all the engines. Between rear coupler of one unit and front coupler of another is no problem, neither is between two front couplers. 

Model and invisible texture - Harold West

Sound effects programming - Stuart Williamson

Additional sounds - Harold Payne

Project coordinator and general instigator - Jim Ward 

Official Invisocar project psychic - Dionne Warwick

EULA

This wagon and sounds are freeware, the end user (that's you) hereby has permission to use, abuse, fold spindle and mutilate, upload to other websites and distribute at will in any freeware add on. Add your own sounds and upload again, maybe this will grow to 100 megabytes.

This software is not to be used in any commercial or shareware venture without express written permission from the authors on penalty of getting your legs broken.

Jim Ward
Chicago, USA
email, jwarde@ameritech.net