S.S. Titanic or R.M.S. Titanic?

We've heard it called "S.S. Titanic". We've heard it called "R.M.S. Titanic". Which is right? This also applies to any other steamship that earns a Royal Mail contract - The Lusitania, the Empress of Ireland, and even the Atlantic!

ss titanic,rms titanic,titanic,ss,rms,royal mail ship,royal mail,royal mail steamer,steamship,steam ship,ss or rms,ss vs rms,prefix,abbreviation,