The term " Amshack " means, at least to me, the small modular shelters that Amtrak installed in many locations in the 1970's & 1980's, rather than pay rent on RR - owned station buildings. Some of these shelters had " Time - Release " doors that unlocked an hour or so before the trains' scheduled arrival time. Several that come to mind were at Hood River & Pendleton, OR.
The current Tacoma Amshack is actually a full - fledged station building, built to an Amtrak standard plan. A similar structure in Anaheim, CA., was recently replaced. ( This was the station built with the roof upside - down !!! ) I recall that Walthers has an HO model of this type of station.