The term "connecting halls" is one I made up, simply because I don't know what these sections of the building are properly called. They are the lower, longer buildings sandwiched between the main hall, and the two extreme end buildings.
The way I developed my WUS model is that it is effectively three buildings, all connected by four panels that serve as these connecting halls. The sides of the East & West Halls, and the sides of the main hall building have thin wooden square dowels painted and glued to the sides, which form grooves for these connecting hall pieces to simply slide into and out of.
The above photo shows how they looked before painting: a lasercut & etched black sheet of styrene, with add-on details of lower ledge moulding, upper ledge moulding, and plastic shirt buttons with the holes puttied in to serve as "ear lobes" for the tops of the pilasters.
This photo shows what one of the front connecting halls looked like before completion. The front pieces were far more complex, as I recessed the palladian windows into arch portals, complete with narrow barrel-vault ceilings. This just shows a placement before I got started on it.
The palladian windows were more of the clear acrylic plastic, laser-cut and laser-etched with the grill patterns on them. Leaving the masking on during lasering left me with a perfect paint mask, and you can see here I used a black paint pen to get black paint inside the etchings before peeling off the masking to form a palladian window.
There are five of these for each of the front and back of each connecting hall, so I had twenty of these to make. The above photo shows five of them after the masking had been peeled back, and one of the face panels of the recessed arch portals set for placement on the far left.
For the rear pieces, I simply glued the windows behind the cutout patterns. This photo shows one of the rear connecting hall units with some of the details affixed, but missing the upper windows.
This photo shows much clearer how the front connecting hall units were created, and here I had one recessed arch portal unit to assemble. Also, the green-paned windows have been installed on this unit.
And here's a terrible photo of one of the front panels installed for placement.
This photo shows both the front & rear connecting hall panels installed in the West Wing, but with West Hall building removed. You can really see the difference between the front & rear panels here, with the recessed arch portals of the front panel eating another 1.1" or so of real estate, and actually clashing a bit with my nice elliptical opening into the main hall. This was an oversight during the planning stages, where I neglected to compensate for this offset when centering the openings. The same thing happened with the openings into the East & West hall buildings. It's noticeable, but not egregious, and I think I can buffer the mismatch with some trim, columns, or even potted trees.
The glass canopies in the roofs of each connecting hall are another story - and provided enough challenge to warrant their own section, so be sure to check that out if interested. Here's the link: http://www.jcstudiosinc.com/Trains/UnionStation/Construction/Canopies/UnionStationConstructionCanopies.html
The connecting halls were tedious - at least as hard, if not harder than the rest of the station due mostly to the individual barrel vault ceilings. These were implemented by boiling 1/8" plastic in water, and strapping them over a soup can while cooling to preserve the approximate shape. This took a LOT of experimentation, and frankly, were I to do this again, I'd use thinner plastic - like 0.06" - which should make the bending easier.
These connecting panels finally gave the station it's long-awaited full-bodied profile. No longer a disembodied collection of buildings, the silhouette of the station could finally be appreciated with these in place.
It was now time to focus on the roofs, canopies, balusters, and other details to bring the whole station to completion.