Railtie.rb snippet from Rails 4.2.5.2(master)

railtie.rb

module Rails
  class Railtie
    # Removed some code
    class << self
      private :new
      delegate :config, to: :instance

      def inherited(base)
        unless base.abstract_railtie?
          subclasses << base
        end
      end

      def rake_tasks(&blk)
        @rake_tasks ||= []
        @rake_tasks << blk if blk
        @rake_tasks
      end

      def console(&blk)
        @load_console ||= []
        @load_console << blk if blk
        @load_console
      end

      def runner(&blk)
        @load_runner ||= []
        @load_runner << blk if blk
        @load_runner
      end

      def generators(&blk)
        @generators ||= []
        @generators << blk if blk
        @generators
      end

      def abstract_railtie?
        ABSTRACT_RAILTIES.include?(name)
      end

      # Since Rails::Railtie cannot be instantiated, any methods that call
      # +instance+ are intended to be called only on subclasses of a Railtie.
      def instance
        @instance ||= new
      end

    protected

    delegate :railtie_name, to: :class

    def initialize
      if self.class.abstract_railtie?
        raise "#{self.class.name} is abstract, you cannot instantiate it directly."
      end
    end

    def config
      @config ||= Railtie::Configuration.new
    end

    def railtie_namespace
      @railtie_namespace ||= self.class.parents.detect { |n| n.respond_to?(:railtie_namespace) }
    end
  end
end