Hi Swdw
Personally, I'd love to see quilling of whistles available, however both major options that are available for programmers to implement this in games are not very good IMO. You're talking about an analogue effect, being replicated with digital files.
The first method available is pitch shifting. This is where you take a looped sound file, and pitch shift it (essentially like playing it faster or slower), to give a higher or lower pitch sound. The problem here is that it only works for quilling a whistle with minimal change, as it only makes it sound deeper or higher pitch. So for a 5 chime whistle that tends to 'climb' through notes, it won't sound right. It also fails to take into account where in the whistle's range the sound was recorded; although tags could take account for this with suitable tags. This is the method used by some of the DCC manufacturers that I've experience, and is the easiest for creators to implement (it would theoretically work with existing 2-part and 3-part horns, but won't be perfect on everything).
The second method is to allow the hornsound creator to provide multiple looped files, and a 'step' file between them. This is generally the option that has the better chance of sounding better (to a degree at least), but it is much much more difficult to create the hornsound for as you will need to be able to record the whistle in a specific way, and it would greatly limit how many 'levels' of quill you have (you'll notice in the video you linked, the 'quill' is about 3 or 4 levels of quill, with an obvious step between them). The better the recording set, the more levels you could theoretically have, but you will need to record a lot of sound files (you would generally need a file for the step up between each quill, plus a loop for each quill, and then a file for each step down). This would absolutely require you to have access to a locomotive with the whistle fitted, and a relatively quiet environment to record it in, and be allowed to blow the whistle at the required levels.
Potentially you could combine these, but generally pitch shifting of the looped files would make having the step files difficult (as you cannot guarantee that the pitch shifted loop will match the step files), so generally this results in just jumping between pitch shifted loops (see steam enginesounds in Trainz for this method).
Personally, I'm a bit of a fan of the ESU methods of whistle creating for DCC, having played with it a bit, and maybe one day we might be able to achieve something like this in Trainz (in theory, it could possibly be scripted, but I haven't actually tried it yet), through a procedural sound system. On a whistle set that I created for one of my HO models, I have it randomly select between 2 whistle sounds when you 'long blow' the whistle (ie hold the button for more than about 1 second), when you let go of the button one of the whistles then has two 'end' options (one has a bit of a quill on the end, one just has a plain end). It was also setup that if you tap the whistle, it randomly selects one of five short 'pop' blasts on the whistle. The short pop whistle is definitely something I'm thinking of how I might script that in Trainz (I have ideas, just not had time to actually put it together
), and at some point it might be fun to try making the random long whistle system work but that might not work as neatly as a standard 3-part horn in Trainz. But of course, this doesn't really provide for a true quilled whistle, it's just fixed whistle quills.
But in the end, any functional 'quillable whistle' (be it fixed quill sounds like TCS's WowSound decoders, or variable using the two methods I mentioned) would still require the content creators to provide suitable whistle sounds, and to support those with their locomotives and cab views. So even if a method were implemented, only content designed to support that feature would generally include it.
That isn't to say we won't potentially look into options in future, but it is a fairly major feature to introduce that would require content creator support to get use from.
Regards