I just eyeballed it but the abutments are approximately the same shape as trestle bents their heights would be. With that in mind .. the area between the bents can’t have a structure. I indicate this with the translucent blue color. That leaves the area between the bents and the road for the little building I wanted. It’s a LOT smaller then I had thought .. darn it.
I colored the area I decided on with red. Half the building on either side of that step. I figured this would just look .. interesting.