I just watched a BigRigTravels or Big Rig Travels video called "U.S. Highway 287 going though downtown Amarillo" (or something like that) also U.S. 87 and U.S. 60. (The same road) I got to see the trench (northbound carriageway, either Buchanan or Filmore) that goes under the edge of the rail yard. It looks very nice and provides an example of what the rest of the highway would like if it was trenched. It's too bad. If they are going to dig two of the carriageways under a rail yard (and the other two carriageways are bridges going over the railroad) instead of bypassing the rail yard, then they ought to have trenched everything else.
This is presuming there were always plans to build an Interstate through the middle of downtown Amarillo when that was never the case. The downtown was there already. The super highways came later.
Then there is the matter of cost. It's one thing to make a pair of cut and cover tunnels for just a couple blocks under a rail line. It's an entirely different thing to build much longer cut and cover tunnels through the downtown area of a modest sized, not major, city.
It looks like when they built the bridge over the rail yard, it appears they actually built the bridges ON TOP OF BUILDINGS!! This is the first time I have seen that. Do they really build bridges on top of buildings?! So if they really do things like that, then they could extend the bridges over the rest of the 20 of 22 streets. (1st and 2nd street have southbound bridges going over them.)
East Pierce Street (US-60) goes over one single story building just North of East 1st Avenue (a BNSF office for the rail yard). The bridges in downtown Amarillo don't span over the top of multiple buildings. Also, it's not clear which was built first, the bridge or the building. By the way it's worth mentioning the Taylor Street and Pierce Street bridges don't meet current Interstate standards.
In decades or centuries past it was more common for road or rail bridges to span over the tops of buildings. Just go to Brooklyn (in person or via Google Street View) and you'll see huge but historical examples of this. It's very rare for any new highway project to have any bridges spanning over existing buildings or property.
One recent example is the re-built interchange with TX-114 and TX-121 in Grapevine, TX. There's a really long fly-over ramp that connects TX-114 East to TX-121 South. That ramp goes over the top of a Chevrolet dealership parking lot. The bridge has really high chain link fences on both sides for the portion that spans that commercial property -probably to keep any crap tossed by motorists from falling onto vehicles in the parking lot below.
None of this creates any precedent to build Interstate bridges or tunnels for I-27 through the middle of downtown Amarillo. The heights of the existing buildings and buildings that were there in downtown Amarillo previously would prevent building over the top of buildings. Bridges could only be built over existing streets. Current politics and urban design trends would make such a thing very difficult to build. Tunneling to modern standards is just too expensive. For the price of tunneling I-27 through downtown Amarillo most of an I-27 extension down toward San Antonio could be built. We're talking billions of dollars for either idea.