Rails core - Jul 2014 - Why are Rails::Application < Rails::Engine < Rails::Railtie singletons but not Singletons?

I was interested to see why Rails.application etc. is a singleton object, 
as I was curious about the use-case for this somewhat strange 
design-pattern in a dynamic language like Ruby-- the object encapsulates 
state, but that state is global, so is it really state?... 

I looked into the Rails source from master back through 3.0, and was 
surprised to see that even though there is an instance method that caches 
the new object, and :new is made private (per the pattern), the singleton 
library itself is not used... and yet it is used elsewhere in the codebase.

I think this is work that Yehuda, Carl, and José mostly did in 3.0 that has 
mostly been shuffled around a bit.

Its origins appear to be the change from every Rails app having a 
Rails::Application to everyone subclassing Rails::Application and it then 
becoming 'illegal' to directly instantiate a Rails::Application.. and
yet..

Anyone have any insights into this?  Or should I ask on stackoverflow? :)

My initial impetus for looking into it, just now, though I'd been thinking 
about it occasionally, was Aaron's recent talk at BigDot Ruby on 
benchmarking performance, which got me thinking about object allocation and 
the Rails singleton.  https://www.youtube.com/watch?v=d2QdITRRMHg

-Benjamin

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