diff --git a/railties/lib/rails/initializer.rb b/railties/lib/rails/initializer.rb
index 44d04688c8b9a2e90a05c34cb0b4dcc1785d63dd..44101dcc948526438808a927822335b16097527e 100644
--- a/railties/lib/rails/initializer.rb
+++ b/railties/lib/rails/initializer.rb
@@ -1,6 +1,6 @@
 require "rails" # In case people require this file directly
 
-RAILS_ENV = (ENV['RAILS_ENV'] || 'development').dup unless defined?(RAILS_ENV)
+RAILS_ENV = (ENV['RAILS_ENV'] || ENV['RACK_ENV'] || 'development').dup unless defined?(RAILS_ENV)
 
 module Rails
   class Initializer
@@ -14,4 +14,4 @@ def self.run(initializer = nil, config = nil)
       end
     end
   end
-end
\ No newline at end of file
+end