Here's the arch that we're working on:
Here's the Sanborn map that shows the general arrangement inside the factory. Obviously, I won't have room for all of this. But a single siding hidden in the wall will be enough. The plan is to leave this portion of the wall open, with the factory (or portions of it) a removeable front to access the track if needed.
Here's another view of the entire front of the factory. I will have to compress it slightly, but it will still be the dominant feature behind New Britain Yard.
It appears that the geometry will allow me to penetrate the building and hide a siding in the wall. (I'm not sure why the pictures have the moire pattern on them).
It looks like to the left of the section with the arch, the factory will continue for another 37 window bays.