Is It Possible To Use Trains On Different Routes?

I heard it is possible to use trains on non-native routes (e.g. 1972 Stock on Sand Patch Grade). However, when I try to do this on the scenario designer, the only options are native trains, and I can't find an option to enable non-native trains. Is it possible to do this, and if so, how?

 

1. While in the scenario designer, press "C" to "Set Scenario Details".
   
2. Click on the "Off the Rails" option.
   
3. Confirm.

Now you can use anything on the route.