Installing Rails on Ruby in the RP+

Only you should follow this tutorial. When it says "Kick off the lonnnnnng lonnnng process with the shell command below:" it's really true. You have to wait a couple of hours.
Later you can install a Dynamic Update Client (like "No-IP") to access your web page from the internet.
But most important thing is that if you run WEBrick server (default for Rails) as your web server you have to specify the listening IP address of your RP+ hardware:

rails server -b 192.168.0.xxx -p 3000

WEBrick by default doesn't listen all interfaces.