Just provide the executable for railtie. This should be fine since Rubygems now warns before overriding them.

bin/rails

@@ -1,7 +0,0 @@
-#!/usr/bin/env ruby
-
-if File.exists?(File.join(File.expand_path('../..', __FILE__), '.git'))
-  railties_path = File.expand_path('../../railties/lib', __FILE__)
-  $:.unshift(railties_path)
-end
-require "rails/cli"

rails.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   s.homepage          = 'http://www.rubyonrails.org'
 
   s.bindir             = 'bin'
-  s.executables        = ['rails']
+  s.executables        = []
 
   s.add_dependency('activesupport',  version)
   s.add_dependency('actionpack',     version)

railties/bin/rails

@@ -1,2 +1,7 @@
 #!/usr/bin/env ruby
+
+if File.exists?(File.join(File.expand_path('../../..', __FILE__), '.git'))
+  railties_path = File.expand_path('../../lib', __FILE__)
+  $:.unshift(railties_path)
+end
 require "rails/cli"