How can I install different versions of Rails and keep the existing ones?

I had Rails 2.3.5 installed, and wanted to upgrade to 2.3.10 as a stepping stone to Rails 3. I thought running gem install rails -v=2.3.10 would install 2.3.10 and keep 2.3.5 as well. But now when I do rails -v, it only lists Rails 2.3.10. How can I install different versions of Rails and keep the existing ones?