Hello,
the problem is that the turntable isn't a real piece of MSTS track, but just a scenery object. The drivable track is covered by the turntable. So you can drive into the roundhouse although the turntable isn't pointing at your track. I don't know if the track the turntable is pointing to is also drivable. If it is there would be a crossing under the turntable. If it isn't drivable (You'll have a derailment.) you'll have to turn the object "turntable" in the RE to make it point to the track you're using at the moment.