The routes.php file should always be there.
In order to add new route files you could do the following:

1. edit the file app/start/global.php
2. add new route files:

• require app_path().'/routes/users.php';
• require app_path().'/routes/users.php';

Checkout this http://laravel-tricks.com/tricks/laravel-5-multiple-routes-files

You can do this by including your route file in app/Providers/RouteServiceProvider.php. Assuming your all routes file in app\Http\Routes directory. put your admin.php file in

• app/Http/Routes/admin.php

Now resister this route file in map method of RouteServiceProvider.php .

public function map(Router $router)
{
    // Authentication
    $router->group(['namespace' => $this->namespace], function($router)
    {
        require app_path('Http/routes.php');

        //custom route file here
        require app_path('Http/Routes/admin.php');
    });
}

Here I have build a package to manage laravel routes. You may check this https://packagist.org/packages/thedevsaddam/multiple_route

With Laravel 5.2, you can do it with this way:

1. Create your router file in "app/Http" directory
   ex: "app/Http/my_route.php"
2. Open file "app/Providers/RouteServiceProvider.php"

• Looking for method "mapWebRoutes(Router $router)" at the end of this class
• Add a statement like this: " require app_path('Http/my_route.php'); "
  Now you can enjoy your new router file. Have fun!

I wonder how you can do this Laravel 5.3?