Jeannette,
In another thread I thought you had found your grandparents return from Argentina. Is it possible that they returned on the same ship that they departed from NY on; or at least a ship belonging to the same Shipping Line? - What ship did they return on and what were their names?
As advised above Titanic was a trans-Atlanitc liner.
For 1912 she was due to sail from NY on Saturdays: 20 April, 11 May, 1 June, 22 June, 13 July, 3 August, 24 August, 14 Sept, 5 Oct, 26 Oct, 16 Nov, 7 Dec, 28 Dec.
She was due to sail from Southampton on Wednesdays: 1 May, 22 May, 12 June, 3 July, 24 July, 14 Aug, 4 Sept, 25 Sept, 16 Oct, 6 Nov, 27 Nov, 18 Dec.