foo.js

var express = require('express');
var router = express.Router();

// my few includes
require('./foo_1')(router);
require('./foo_2')(router);

module.exports = router;

foo_1.js

module.exports = function(router){
  router.get('/one', someFunction);
  router.get('/two', someOtherFunction);
  router.get('/three', someFunctionHere);
}

foo_2.js

module.exports = function(router){
  router.get('/four', someFunctionAndHEre);
  router.get('/five', someFunctionYesHereToo);
}