What does it mean for a programming language to be "on rails"?
I'm currently working with Groovy and Grails. While Groovy is pretty straight-forward since it's basically Java, I can't say I grok Grails. I read that Groovy is to Grails as Ruby is to Ruby on Rails, but what does that mean?
Answer
To address your confusion with the metaphor (though it has been answered in other words under your question):
Groovy is to Grails as Ruby is to Ruby on Rails, but what does that mean?
Grails was a web framework built on/with the Groovy programming language to do the same thing for Groovy that Rails (a web framework for Ruby) does for Ruby.
What does it mean to be "on rails"?
The answer to this comes down to the essence of these web frameworks.
These web frameworks (Grails & Rails) are built on the premise of "convention over configuration", which means that using common conventions to develop web applications can lead to higher productivity and more maintainable applications (this is a gross generalization). And by defining a convention and sticking to that you will find that your applications are easy to generate and quick to get up and running.
This is what it means to me to be "on rails", just like a train. When a new train route is developed there is no worry about reinventing the way the train will get from one place to another, it's been solved by a single convention for decades: rails. Just as the tracks on a train route constrain its path from two locations, convention-based web frameworks use conventions to the flexibility of application developers so that they can concentrate on what the essential business problem of their application.
One key benefit of a convention for a web framework is that the web framework can now make assumptions on how certain layers of the application hook together. In Rails, one can usually assume that if one's database table has a plural name, the ActiveRecord class mapped to that table will have the corresponding singular name. Consequently, Rails code generators can consume the data-mapping information to generate data access code such as dynamic finders, migrations, lazy-loaded association traversals, etc. This data access code in a configuration-based framework is laborious to code by hand.