config.ru

require 'rubygems'
require 'bundler/setup'
require 'yaml'

require 'action_dispatch'
require 'action_controller'
require 'action_view'

routes = ActionDispatch::Routing::RouteSet.new

routes.draw do
  get '/' => 'mainpage#index'
  get '/page/:id' => 'mainpage#show'
end

class MainpageController < ActionController::Metal
  include AbstractController::Rendering

  include ActionView::Rendering
  prepend_view_path('/path/to/templates')
  include ActionController::Rendering
  include ActionController::ImplicitRender

  def index
    self.response_body = "<h1>Front Page</h1>"
  end

  def show
    self.status = 404
    self.response_body = "<pre>#{env['action_dispatch.request.path_parameters'][:id]}</pre>"
  end
end

#use ActionDispatch::ShowExceptions, ActionDispatch::PublicExceptions.new("public/")
use ActionDispatch::DebugExceptions
run routes

Gemfile

#ruby '2.1.0'
source 'https://rubygems.org'
gem 'rack', '~>1.5.2'
gem 'actionpack', '~> 4.1.0.beta1'
gem 'railties', '~> 4.1.0.beta1'