Make sure the port your URL is mapped to is the same port your application is listening to (8000 it seems?).

Port is already set to 8000

For this error, you'll need to add CORS headers to allow your frontend URL to access the API.

When I add CORSMiddleware to my FastAPI app it causes 502 errors. My frontend is at https://gregarious-celebration-production-d2a0.up.railway.app and my backend is at https://dirtbike-tuning-hub-production.up.railway.app. How do I add CORS headers without breaking the app

Cors middleware is not related to the 502 errors you're getting. It mostly stems from mapping your URL to a port your app isn't listening on.

If your app is already listening on port 8000, map your URL to 8000 as well. You can find it in Service Settings > Networking.

I changed the port to 3000 and it is running the backend now. Not sure if this is a true fix but its a move in the right direction

The 502 was caused by a port mismatch. The CORS was a side effect of the port mismatch.