Single file Rails application

application.rb

require "action_controller/railtie"

class Application < Rails::Application
  config.session_store :cookie_store, :key => '_rails_session'
  config.secret_token = '095f674153982a9ce59914b561f4522a'
end

class PagesController < ActionController::Base
  def index
    render :text => "Hello, world!"
  end
end

Application.routes.draw do
  get '/' => 'pages#index'
end

config.ru

require './application'
run Application