I don't know that its been fixed, but I generally stay far away from AI Steam because the AI firemen can't fire a boiler worth a ****. Expect to hear "PFFFFFFFFT" as the safety valves will rarely close any time you're near it. There are things you can do to negate this (Short sessions help), but last I knew this problem still occurs. Drives me nuts....
Falcus
There's a relatively quick and easy way to fix the constant hisssssssssssssssing of the safety valves under AI/DCC, but you'll need to clone and then edit the e-spec (and then cloned and edit a loco, as well.)
You're basically have to
dumb down the e-spec since you can't make the AI drivers
smarter!

(Unless you want to create a new e-spec, from scratch.)
Here's how to do it: In CM, created a cloned copy of the e-spec. Then change the name of it -- e.g., change the "username" in the config file to indicate its an e-spec made for AI/DCC. For example, change the name from "USRA Light Mikado" to "USRA Light Mikado AI/DCC".
Then lower the value of "max-fire-temperature" under the "steam" section until the safeties stop popping off. You may have to experiment a little -- you may have to lower it 100, 200, or more degrees, but doing so won't affect performance of the loco under AI/DCC -- Trainz only pays attention to the "motor" settings under AI/DCC even for steam locos in regards to pulling power, acceleration, top speed, etc.
If you try using that cloned e-spec in cab mode, it'll probably perform poorly -- but under Ai/DCC it'll work fine and the hissssssssssing of the safety values will stop. So, you'll next want to create a cloned version of the locomotive that uses this "special" AI/DCC version of the e-spec as well, and name that loco accordingly so you know it's set up to be used under AI/DCC control.