development.rb

config.middleware.use RoutesReloader

routes.rb

class ActionDispatch::Routing::Mapper
  def draw(routes_name)
    instance_eval(File.read(Rails.root.join("config/routes/#{routes_name}.rb")))
  end
end

BCX::Application.routes.draw do
  draw :api
  draw :account
  draw :session
  draw :people_and_groups
  draw :projects
  draw :calendars
  draw :legacy_slugs
  draw :ensembles_and_buckets
  draw :globals
  draw :monitoring
  draw :mail_attachments
  draw :message_preview
  draw :misc

  root to: 'projects#index'
end

routes_reloader.rb

# updated version of @shime's solution from https://gist.github.com/dhh/2492118

class RoutesReloader
  ROUTES_PATH = Dir.glob("config/routes/*.rb")

  def initialize(app)
    @app = app

    @routes_reloader = ActiveSupport::FileUpdateChecker.new(ROUTES_PATH) do
      Rails.application.reload_routes!
    end
  end

  def call(env)
    @routes_reloader.execute_if_updated

    @app.call(env)
  end
end