For what it's worth, I have added the LGB electric sound module to two of my LGB Electric locos controlled with Railpro.
You don't get to ring the bell or toot a horn , (unless you use a track magnet and the LGB track pickup module that plugs into the sound module)
But you do have good rev up and running sounds.
You will have to adjust the speed setups, etc, with your controller to match.

Yes. The best response is from actual switches Vs some of the optical sensors which require different wiring.

Don't expect it to chuff like a Phoenix though. It seems Ring is using the input to suggest the speed to the program. If you start fast or stop fast the chuff will lag, or continue after you stop!  I've found if I'm careful to start and stop slowly that it isn't as bad. It's been suggested that adding Load will help that. I prefer to run with no load, having absolute control over speed for switching.

I really really doubt others have done that, and if anyone actually has I'd love to read how they did it. What I think you'll actually find it that people have made a simple single motor rpm loop, like a horn file: Startup -> Single rpm -> Shutdown. To me and I guess most others not what we'd call a prime mover/engine sound as it doesn't have a range of rpm sounds.

So, if your wanting a prime mover/engine sound with a range of rpms (think notches) then my first reply still stands. If you can provide otherwise that we can make these please do so here and I'll stand humbly corrected.

If you just want a single rpm engine sound then yes, that can be done by us users. No one has posted or made reference to such a sound to this forum before.

- Tim

Oh we have thoughts, the same thought as you have (had) as you've asked this type of question before and received the one universal answer but it appears that you have forgotten it so to reiterate "No one other than Ring themselves can make prime mover/engine sounds for RailPro" so unfortunately no, no one here will have a sound file for a railbus.

As for 892 views and no thoughts from anyone, probably a good 90 percent plus of those views are bots and internet randoms and not RPUG members. For the remaining views from RPUG members a good probable 90 percent don't or hardly ever post. That leaves relatively few of us who do post and we do sometimes forget to come back to questions.

- Tim

Note that the LM-3S is limited in the voltage not just amperage when trying to drive G scale locos.
If the railtruck requires more than 14-16 volts to get to the speed you require, then you need to move up to the LM4S.

If the G scale loco never requires the full 1 amp of the LM-3S limit and around 14 volts gets you up to full speed for that loco, go for it.