# yum search rails
# yum install rubygem-rails mysql-devel ruby-devel
... rubygem-rails is in the 'epel.repo'.
# gem install rails
... and wait 5 minutes for it to complete the install.
# gem install mysql -- --with-mysql-include=/usr/include/mysql --with-mysql-lib=/usr/lib/mysql --with-mysql-config
which is the first hit, by Googling .. dag rpm rails ..
( The page was issued 2005, at that time there was no
rpm package(s) rubygem-* available.)
Please also do # yum search ruby
to see some other 'rubygem-xxxx' packages.
Good luck !