The freeware G-TraX Husky stacks and the ones in Stevens Pass are similar but not identical. For example, the StP stacks have EOTDs, the reporting marks are GNTX and BNSF (vs. DTTX and CRLE), and the container company names are fictionalized (per RSC policy). In other words, they're not interchangeable. If you don't have the route, you don't have the upgraded version.
The paths are:
Assets\G-TraX\Trains\RailVehicles\Freight\DoubleStack
Assets\GNTX\StevensPass\RailVehicles\Freight\DoubleStack
Hope that helps.