Reloading express routes/middleware on file change WITHOUT restarting node process.

server.js

if ( process.env.NODE_ENV === 'production' ) {
  // dev runs with babel-node so this isn't necessary
  require('babel-core/register'); // this includes the polyfill
  require('newrelic');
}

import 'source-map-support/register'

import Debug from 'debug'
import express from 'express'
import request from 'request'
import logger from 'morgan'
import cookieParser from 'cookie-parser'
import fs from 'fs'
import path from 'path'
import compression from 'compression'
import favicon from 'serve-favicon'
import swig from 'swig'

import chokidar from 'chokidar'
import webpack from 'webpack'
import webpackDevConfig from '../../build_config/webpack.dev.config'
import webpackDevMiddleware from 'webpack-dev-middleware'
import webpackHotMiddleware from 'webpack-hot-middleware'

const webpackDevCompiler = webpack(webpackDevConfig);

const debug = Debug('server');
const server = express();

// setup webpack middleware and server-side hot-reloading for development
if ( process.env.NODE_ENV === 'development' ) {
  server.use(webpackDevMiddleware(webpackDevCompiler, {
    noInfo: true,
    publicPath: webpackDevConfig.output.publicPath,
    stats: {
      hash: false,
      version: false,
      timings: true,
      assets: false,
      chunks: false,
      modules: false,
    }
  }));
  server.use(webpackHotMiddleware(webpackDevCompiler));

  // load middleware
  server.use(function(req, res, next) {
    require('./middleware')(req, res, next);
  });
  
  // load routes
  server.use(function(req, res, next) {
    require('./routes')(req, res, next);
  });

  // Do "hot-reloading" of express stuff on the server
  // Throw away cached modules and re-require next time
  // Ensure there's no important state in there!
  chokidar.watch(__dirname, {ignored: /index\.js$/})
    .on('change', (path) => {
      debug(`Clearing ${path} module cache from server`);
      if (require.cache[path]) delete require.cache[path];
    });
}

if ( process.env.NODE_ENV === 'production' ) {
  server.use(logger('combined'));
} else {
  server.use(logger('dev'));
}

// gzip responses
server.use(compression());

//
// TEMPLATE SETUP
//
// assign the swig engine to .html files
server.engine('html', swig.renderFile);
// set .html as the default extension
server.set('view engine', 'html');
server.set('views', path.join(__dirname, '../html'));

// Swig will cache templates for you, but you can disable
// that and use Express's caching instead, if you like:
server.set('view cache', true);
// To disable Swig's cache, do the following:
swig.setDefaults({ cache: false });
// NOTE: You should always cache templates in a production environment.
// Don't leave both of these to `false` in production!

server.use(favicon(path.join(__dirname, '../../public/favicon.ico')));

// return static files if found
const oneYearInMilliseconds = 31536000000;
server.use(express.static(path.join(__dirname, '../../build_public'), {maxAge: oneYearInMilliseconds}));
server.use(express.static(path.join(__dirname, '../../public'), {maxAge: oneYearInMilliseconds}));

server.use(cookieParser());

server.set('port', process.env.PORT || 3000);

// start server
server.listen(server.get('port'), function () {

  const env = server.get('env');
  const port = this.address().port;

  debug('Express server running Rentals ' + server.get('env') + ' on port %s', port);
  if ( process.env.NODE_ENV === 'development' ) debug('Please wait, Webpack is compiling…');
});

Do you have the webpack.dev.config file?