Well since you asked.
The Test Track feature was promised and funded in the kickstarter for TANE. As part of the TIG group I submitted replies to questions for things that might be included for testing steam locos. So I know steam locos were initially included in the development. I even have a copy of a video distributed by N3V of an early version of the Test Track with N3V employees discussing this new feature and guess what - the video shows a steam locomotive being tested on the track.
So what happened. After TANE came out. I initially asked, "Why can't the test track handle steam locos?". I think the reply was it needs more time so maybe later. Like Paul, I checked every new version and update to see if steam locos had at long last been included. And like Paul I eventually gave up checking. I have definitely given up hoping N3V will live up to their commitment I thought I had back in the kickstarter development. There was no discussion at that time that including steam was on the extras list and it was not funded. It's definitely a sore point with me.
Ann's point that steam especs are more complicated to develop and therefore including steam locos would be a programming nightmare I really don't buy into. Besides it lets N3V off the hook too easily. Sorry I do respect you and your work immensely and up to this point you've always been right.

But I see N3V's Test Track basically as recording the values of various engine and train parameters at short intervals as the test is run and plotting the results and optionally writing them out to a text file for whatever use a user to make of it. The way I see it the only difference between diesel and steam loco tests is in the number of parameters recorded.
I have written Trainz game scripts that record 17 so parameters for steam loco tests at 1 to 2 sec intervals and I have to do more calcs than N3V would to just record them because I don't have access to the underlying native code and have to calculate some intermediate things just to get the value of some of these parameters. I don't see it being all that difficult since most everything I record has to be calculated by the native code and just needs saving and presentation the same as with the diesels.
I will say my plotted results for the (few) diesel tests I conduct come out as fairly smooth curves similar to those displayed for the diesels on the Test Track and agree pretty closely to the results displayed there.
The plots of my results for steam locos are however anything but smooth curves. When post processing the results that show the most fluctuations I apply averaging functions to average out data over several time intervals representing 5 to 15 seconds in real time.
In addition for most steam locos the recorded power output from my tests is excessive, exceeding the loco's rated power considering boiler pressure, cylinder dimensions and drive wheel diameters when running at low speed. Another problem is the difference in power output that Trainz shows for steam locos between running in the forward and running in reverse directions. Steam locos running in reverse show significant power reduction compared to running in the forward direction for the same operating conditions. In this case the output is only around 60 - 70% of that produced in the forward direction. So I don't see a good simulation of steam physics in my test results.
I was hoping N3V's test track would confirm my results or show where I was wrong but there's no possibility of that if you can't test steam locos on the track.
Bob Pearson
PS
I haven't really seen anyone else ask this before.
I guess you missed all my posts that I posted after each new SP was released for TANE pointing out that steam locomotives are not allowed on the Test Track.