Outside, I use a Bridgewerks Mag 15 and a UR 15 throttle. The unloaded peak potential is close to 31v input! Of couse, under any load at all it drops to 24v or less. Your mileage will vary since you don't necessarily have the same 115 hz current I have on the outside of the house.
However that is not how I normally run the system. I use the throttle to set the track voltage to 19v. My meter confirms what the MTH remote tells me - that the track voltage is 19. Then, if no one is running a non DCS locomotive, I do not touch the throttle from that point. I just turn the whole system on from the circuit breaker. (or the wall plug depending on how lazy I am)
If the track voltage is 19v the input voltage to the TIU is about 2v higher - so 21 volts. Yes, I could run it at 24v all day long. But there is no need to do that. I like to have the track voltage at that level because it wont burn out the passenger car lights. (and they dont look too bright) At about 21v input and 19v output, the light blub seems to provide an reasonable filter level without affecting anything else. I picked this bulb on purpose for the 28v input. Runing at 21v, it will last 100s of human lifetimes. My signal strenght all around the 300 foot track it 10. So the light bulb doesn't hurt.