The short answer: You need client-side scripting to make real-time games possible.
The long answer:
What is a server?
A server is a computer used to run services for other computers on a network.
What is a web request
A web request is what web servers handle. It is a request, often created by a user from home on a home computer, netbook or a smartphone, generally asking for a certain page. The server receives this request, runs the script (unless a static html file is requested) and returns a response that is normally a html file. The nature of this request is that it is like a short handshake, it happens and then it's over.
What is Ruby on Rails?
Ruby on rails is a scripting language, designed to run on a server and handle web requests. This is used to design web application much like this website.
Meaning, I want to read page:
Is it possible to make/translate a 3d engine to ruby on rails?
The server(s) at the domain gamedev.stackexchange.com take my request and send me back the textual data I wanted formed into a html file that my browser can render. They run some sort of server-side script to collect all the pears of wisdom related to this question and organize them. This kind of script could be written in Ruby on Rails.
What is a 3d engine
It is software that provides real-time 3D rendering capabilities - if you ever played any of these 3d games you know that unlike a short handshake the games provide a constant steam of imagery directly to your monitor.
This is very expensive in processing power, namely in GPU power. Most modern games demand enough graphical processing power to occupy one modern computer*.
You do not want to pay for that power nor do you have the software tools to attempt something like this. Ruby on Rails was not designed for that purpose. It is designed for the short handshake situation where the server sends one page and forgets about the client while moving on to the next request.
3d game engines run on the home computer, you RoR never runs or gets near a home computer. You need client-side scripting to make real-time games possible. You could start with Unity or ActionScript with Stage3D if you have some aversion towards webGL.
To be honest, if porting a 3d engine to some language is practical, someone has already done it and a 3d engine that is used with that language already exists.