Terminal stuck on installing ri documentation for rails

Are you sure it's stuck? The documentation takes ages to install (upwards of 4 mins even on a high end MBPr). You could also install the rails gem without documentation using:

gem install --no-ri --no-rdoc rails

This has happened to me a few times. I believe it is a known issue which has never been resolved properly.

Simply install the gem without the documentation, if you don't mind, like the other answer suggests.

gem install rails --no-ri --no-rdoc

UPDATE (2020-04-29)

The new way of generating no documentation is adding the --no-document (in short -N) option, like so:

gem install rails --no-document # or -N in short