The answer to 'sewer line rusty water' is that rusty water in the sewer line is usually caused by corrosion of the metal pipes. This can happen over time as the pipes age and deteriorate. Symptoms include reddish-brown or orange-tinted water coming from drains and fixtures, as well as possible clogs or backups in the sewer line.
The best solution is to have a professional plumber inspect the sewer line and determine the extent of the corrosion. They may recommend replacing the affected section of pipe, which is typically a job for a professional due to the complexity of working on underground sewer lines.