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The "RMS" designation stood for "Royal Mail Ship" (or Steamer) and was applied to vessels that carried mail for the British Royal Mail postal service. The first designations were assigned in the 1840's and it was not used exclusively for transatlantic liner services.
Wikipedia has a full list of ships that carried this prefix and it appears from their list that there was never a RMS Heroic (although RMS Homeric comes close).