ASP.NET MVC User routing like that in StackOverflow?

I've looked at the routing on StackOverflow and I've got a very noobie question, but something I'd like clarification none the less.

I'm looking specifically at the Users controller

https://stackoverflow.com/Users
https://stackoverflow.com/Users/Login
https://stackoverflow.com/Users/124069/rockinthesixstring

What I'm noticing is that there is a "Users" controller probably with a default "Index" action, and a "Login" action. The problem I am facing is that the login action can be ignored and a "UrlParameter.Optional [ID]" can also be used.

How exactly does this look in the RegisterRoutes collection? Or am I missing something totally obvious?

EDIT: Here's the route I have currently.. but it's definitely far from right.

    routes.MapRoute( _
        "Default", _
        "{controller}/{id}/{slug}", _
        New With {.controller = "Events", .action = "Index", .id = UrlParameter.Optional, .slug = UrlParameter.Optional} _
    )