Unisntalling rails command needs to uninstall railties, not only rails.
% gem uninstall railties -v 5.0.0.rc1
You have requested to uninstall the gem:
railties-5.0.0.rc1
coffee-rails-4.1.1 depends on railties (< 5.1.x, >= 4.0.0)
jquery-rails-4.1.1 depends on railties (>= 4.2.0)
rails-5.0.0.rc1 depends on railties (= 5.0.0.rc1)
rails-controller-testing-0.1.1 depends on railties (~> 5.x, development)
sass-rails-5.0.4 depends on railties (< 5.0, >= 4.0.0)
sprockets-rails-3.0.4 depends on railties (>= 4.0, development)
twitter-bootswatch-rails-3.3.4.0 depends on railties (< 5.0, >= 3.1)
web-console-3.2.0 depends on railties (>= 4.2)
If you remove this gem, these dependencies will not be met.
Continue with Uninstall? [yN] y
Remove executables:
rails
in addition to the gem? [Yn]
Removing rails
Successfully uninstalled railties-5.0.0.rc1
% rails
rbenv: rails: command not found