Polishing Metal Rail
The metal appears to be copper. There are house hold cleaners for copper bottom pots that will help clean this. Wrights copper cream is a product my Mom uses. You may need a "car buffer" with a cloth bonnet to do a large area. After getting it bright and clean a spray with a clear lacquer will keep it looking nice for several years. Without sealing it will quickly tarnish again. I think lemon juice and baking soda will also work but may take more elbow grease than wrights.