First, to answer the question: init documentation from July 2011 (Rails 3.0 when this question was asked) indicates railties/bin/rails as the first file to run when Rails is called.
Secondly, more generally, this question was asked and answered four years ago. The link in the first answer still works, but only for the current version (Rails 4 at the beginning of 2016). Those seeking historical information on this topic may be most helpful when attempting an online archive. For example, a snapshot of this document from April 2013 (Rails 3.2):
http://web.archive.org/web/20130409080134/http://guides.rubyonrails.org/initialization.html
Ian m
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