This route is sold as a paid DLC by a third party, but a "Lite" version exists for free. It covers 60 miles of canyon running in California. The free version lacks the seasonal textures of the paid one, but the track geometry and rock formations are identical. It is the best American freeware route available.
Arguably the most famous freeware locomotive in Train Simulator history. The Pennsylvania Railroad T1 is a 4-4-4-4 duplex steam engine. The free version features a fully modeled firebox, dynamic braking, and a custom sound set recorded from a real steam chime. It requires effort to drive (it slips like crazy above 80 mph), but mastering it is a rite of passage. Train Simulator Classic Addons Free
They often require to work (e.g., a free livery for a Class 66 requires you to own the original Class 66 DLC). This route is sold as a paid DLC