If Trainz wants to take a major step forward and improve significantly, N3V must focus on improving the up-close detail of routes, as well as the sound. The current sound engine (if there even is one) is extremely basic and likely has not been touched in over a decade except for bug fixes.
What I want is for Trainz to get the native ability to edit and apply effects to sounds. Things such as equalization or reverb/echo. IRL you cannot hear the high pitched whine of a diesel engine at full power from 3 or 4 miles away, but you can certainly sense at least a rumble in the ground, which can be done with equalization, cutting off the higher frequencies at higher distance so that only the bass frequencies are heard. And as that loco (or any sound source) gets closer to the player, the cutoff gets higher so that more and more higher frequencies are audible, the center of the locomotive being the point where the high frequencies are loudest.
Things mentioned, like a sound library that has a selection of common sounds (huge selection of random environmental sounds, such as birds, water, different car/truck sounds, and platform passenger sounds that don't sound like zombies), that can be attached at points on many scenery assets, should be optimized so that they don't necessarily have a big overhead.
Echo and reverb is critical, it gives the sense of space. Hearing my enginesounds without reverb or equalization makes me feel like I am trapped in a box with sound-insulated walls with the loco! And when inside the loco cab, almost always you can hear the lower frequencies of the engine and outer sound, but the higher frequencies are blocked out as these are more damaging to ears. I'm not sure exactly what plays a role in how much a sound echoes but I feel like the bounding box and PhysX or some thing like that could be used to make the (one-time) reverb calculations for a given scene when it is loaded. Although how we perceive reverb and echo could be dependent on how far we are from the sound source, so I don't know how that would work with the new cameras that you can fly all over the place with.
Ability to add a high and a low tone horn. Maybe more so UK oriented but the regular hornsound can be modified so that it can support WAV files with loop points embedded into the file (like the "other" sim).
Startup/shutdown ability, although this would mean some changes to how traincars and trains are handled by the game because of how a single train can only have one throttle setting. Maybe only make the multiple-compatible, connected locos able to be controlled by a single HUD control? Add another HUD control for locos of another multiple working type?
A different falloff for sound as you get farther. Also calculated by how many objects/bounding box are inbetween the sound source and the player.
Stereo sounds.
Slight variations in the pitch/speed at which all sounds are played, unless otherwise noted (in config). A range of pitch bend/variation can also be specified in the config. Maybe half a semitone lower when two or more engines are being run together, this helps prevent the weird audio phase issues that makes the sound come out like a vacuum tunnel (caused by two audio files of the same pitch played simultaneously).
More environmental sounds in general, such as the sounds of cars/trucks/buses added for each individual vehicle, all combining to give off the sound of a motorway, and the sounds of miscellaneous birds and things of the sort.
Different applications/effects on sound under bridges, tunnels, etc. through the use of equalization and reverb and such.
New tracksounds and junction sounds. Better sounds for flange squeal on tight curves, and a different sound for wheelslip (or no sound at all depending on many factors).