Not rails application

require 'telegram/bot'

class WebhooksController < Telegram::Bot::UpdatesController
  def start(*)
    respond_with :message, text: 'Hello!'
  end
end

TOKEN = 'secret:token'
bot = Telegram::Bot::Client.new(TOKEN)

# poller-mode
require 'logger'
logger = Logger.new(STDOUT)
poller = Telegram::Bot::UpdatesPoller.new(bot, WebhooksController, logger: logger)
poller.start

# OR
# rack-app. Better use some router and mount on some secret endpoint.
run Telegram::Bot::Middleware.new(bot, WebhooksController)

Note about considering Rails

Here are some advantages of Rails:

• First of all is hot reload.
• Bootstrap system with configuration files and initializers.
• Autoloading classes.
• A lot of tools and integrations.
• And much more.

While the only disadvantage I see is that it may be to fat by default. However this is not critical: middleware stack can be simply cleared from unused items, and unused railties can be simply removed.

I you still think that using Rails is an overkill for simple bot app, for the last resort you can just use custom config.ru with little stack for production.