If you want to test your app locally in a mobile or tablet, without the need to have a server, you can try ngrok. Ngrok is a very cool, lightweight tool that creates a secure tunnel on your local machine along with a public URL you can use for browsing your local site.
It is very simple to use. Follow these steps:
Download ngrok and install it.
$ rails s
Go to the directory where you extracted the zip file and run it.
$ ./ngrok http 3000
You can now access the Rails app from the URL displayed in the ngrok output. It will be something like: http://2ec640eb.ngrok.io