For Rails6, Rails5 and Rails4 you can do the following:
Put the line below at the end of your routes.rb
get ':action' => 'static#:action'
Then requests to root/welcome, will render the /app/views/static/welcome.html.erb.
Don’t forget to create a ‘static’ controller, even though you don’t have to put anything in there.
Limitation: If somebody tries to access a page that does not exist, it will throw an application error. See this solution below that can handle 404s
For Rails3 you have to use ‘match’ instead of ‘get’
match ':action' => 'static#:action'