A word of caution

There should rarely be a reason for you to split up config/routes.rb. If you need to, probably your whole application should be split up.

Split it anyway

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Rails::Engine looks at config.paths['config/routes.rb'] and registers its value with app.routes_reloader. This means you could put routing files anywhere and then require them. However, I recommend to put any routing files into config/routes/:

# config/routes/example.rb

Rails.application.routes.draw do
 resources :example
end

After creating your routing files, tell Rails how to find them:

# in config/application.rb

# Rails::Engine will register these files with app.routes_reloader
config.paths['config/routes.rb'] = Dir[Rails.root.join('config/routes/*.rb')]

Note that this will neglect your old config/routes.rb (which by now should be empty, anyway). However, you might want to leave a note to other developers:

# config/routes.rb

# See config/routes/*.rb