Friends,
Google can maybe save a bit of work, here.
1) There is a model of St. Louis Union Station in the Sketchup Warehouse (I know Sketchup is not really Trainz compatible, but the model could be downloaded, loaded into Blender, and re-topo'd);
2) Google street view shows all four sides of the current building;
3) There are a number of Panoramio shots taken in the vicinity of the Building accessible from Google Earth;
3) Overall dimensions can be approximated from the ruler function in Google Earth; and
4) There is historic aerial imagery accessible through Google Earth back to 1988.
The appropriate USGS Quadrangle map would give a more accurate estimate of overall dimensions.
Finally, the St. Louis Public Library (or another Library in the area) might have Sanborn fire insurance maps from several different years. As I recall, the Sanborn maps provide dimensions, and show outbuildings.
ns