[SelectActionByParameter(Name = "id")]
public ActionResult Articles(int id)
{
    /* ... */
}

[SelectActionByParameter(Name = "tag")]
public ActionResult Articles(string tag)
{
    /* ... */
}

public class SelectActionByParameterAttribute : ActionMethodSelectorAttribute
{
    public string Name { get; set; }

    public override bool IsValidForRequest(ControllerContext controllerContext, MethodInfo methodInfo)
    {
        return controllerContext.HttpContext.Request[this.Name] != null;
    }
}

public class DocumentsController : Controller
{

   public ActionResult Articles()
   {
       // ...
   }

   public ActionResult ArticlesByNumber(int id)
   {
       // ...
   }

   public ActionResult ArticlesByString(string id)
   {
       // ...
   }
}

           routes.MapRoute(
                "Articles1",
                "Documents/Articles",
                new { controller = "Documents", action = "Articles" }
            );

           routes.MapRoute(
                "Articles2",
                "Documents/Articles/{id}",
                new { controller = "Documents", action = "ArticlesByNumber" },
                new { id = @"\d{1,9}" }
            );

           routes.MapRoute(
                "Articles3",
                "Documents/Articles/{id}",
                new { controller = "Documents", action = "ArticlesByString" },
            );

public class DocumentsController : Controller
{
   
   [Route("Documents/Articles")]
   public ActionResult Articles()
   {
       // ...
   }

   [Route("Documents/Articles/{id:int}")]
   public ActionResult ArticlesByNumber(int id)
   {
       // ...
   }

   [Route("Documents/Articles/{id:alpha}")]
   public ActionResult ArticlesByString(string id)
   {
       // ...
   }
}