A tool to automatically generate a UML diagram for a Rails Application

You could try Rails ERD as a an alternative to Railroad. It is maintained and works specifically for Rails 3.

RubyMine has a tool to do this; they call it a "model dependency diagram", but it generates a UML-like graph for your application models.

If you don't want to use RubyMine, check out RailRoad. It's a standalone tool that does similar, though RubyMine's is prettier, IMO.

For Rails 3, try Railroady, it's maintained as well.