routes in the env via the request object

actionpack/lib/action_dispatch/http/request.rb

@@ -110,6 +110,10 @@ module ActionDispatch
       env["action_dispatch.routes".freeze]
     end
 
+    def routes=(routes) # :nodoc:
+      env["action_dispatch.routes".freeze] = routes
+    end
+
     def original_script_name # :nodoc:
       env['ORIGINAL_SCRIPT_NAME'.freeze]
     end

railties/lib/rails/engine.rb

@@ -520,15 +520,14 @@ module Rails
     def call(env)
       env.merge!(env_config)
       req = ActionDispatch::Request.new env
+      req.routes = routes
       req.engine_script_name = req.script_name
       app.call(env)
     end
 
     # Defines additional Rack env configuration that is added on each call.
     def env_config
-      @env_config ||= {
-        'action_dispatch.routes' => routes
-      }
+      @env_config ||= {}
     end
 
     # Defines the routes for this engine. If a block is given to