Custom top level rack middleware

config.ru

# This file is used by Rack-based servers to start the application.

require ::File.expand_path('../config/environment',  __FILE__)

class MyMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    path = env['PATH_INFO']
    env['PATH_INFO'] = '/hello/peter'
    req = @app.call env
    req[1]['Custom-Header'] = ' Hello Peter'
    req
  end
end

run MyMiddleware.new(Rails.application)