Realty Income (NYSE: O) has been a compounding machine. The real estate investment trust (REIT) has delivered a 13.6% compound annual total return since its 1994 public market listing. With more dividend growth likely, a $25,000 investment could compound into real retirement income. An income compounding machineRealty Income offers investors a high current income yield that should grow over time. It has all the makings of an ideal retirement income investment.