Since railings are a system family, you can’t assign geometry to subcategories to control their object styles, the way you can with normal families. So if you have a railing with cables that are reading too heavily, it can be tricky to adjust only the cables.
One thing you can do is reduce the size of the cable profile – if they’re modeled at 1/4″ in diameter, try changing that to 1/16″.
Another thing you can do is go into Object Styles and change the line weight of Rails (found under Railings). If the smallest line weight isn’t fine enough, you can change the color from black to grey. Of course, this will change the appearance of all rails in your model, which might not be what you want.
If you only want to change the appearance in certain views, try a Visibility/Graphic Override. This is great if the railing you want to change is the only railing in the view.
If you know a better method, please leave a comment!