It looks like there are some problems with access to the far corners--the back wall may be 36" deep (which is pretty deep--try reaching something 3' away without smushing anything in between you and the thing) but keep in mind that the corners will be less accessible--the upper corner 38" deep isn't much worse, but reaching anything behind the roundhouse will require just over 5 feet of reach! I assume that the little indentation in the corner is to make room for something structural in the room--it is not big enough to be an access hole.
Two suggestions:
First, consider putting the UP/BNSF in center foreground on a lower level, and having the tracks behind them on a higher level (there is plenty of space for elevation on either side) so you can run the foreground tracks in a tunnel UNDER the interchange yard in the center. That would allow you to narrow that back section to 2 feet or less.
Second, if you are willing to give up the loop that goes directly around the roundhouse, you could shove the track of the yard inward (to form more of a "teardrop" shape) a foot or so, reducing the potential reach to an only slightly arm-twisting three feet or so.