Drop the railvehicles folder into the following directory and overwrite if prompted:
C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\DSGDDR\RailSimulatorUS

Drop the SF_lightwt and Common folders into the following directory and overwrite if prompted:
C:\Program Files (x86)\Steam\steamapps\common\RailWorks\Assets\G-TraX\Trains\RailVehicles\Passenger

In the consist builder, under wagons, search "SF_L" and the lightweight Santa Fe cars will appear.
Many thanks to G-TraX and associated reskinners for making these cars available to the public domain.