Understanding the different types of addons will help you tailor your downloads to your interests:
To help you get started on your freeware journey, let me know: Train Simulator Classic Addons Free
Dovetail periodically releases free items. To find them: Understanding the different types of addons will help
The most convenient source. It allows one-click installations directly through your Steam client. The file library here is strict about quality control
Excellent repositories for British rail content, offering thousands of community assets, reskins, and routes.
With the recent push of Train Sim World (the "next-gen" version) and the aging engine of Train Simulator Classic, some fear the freeware scene is shrinking.
If you love British Rail (BR Blue, Sectorisation, or modern Network Rail), this is your Mecca. The file library here is strict about quality control. You won't find broken links or unfinished models here.