In 泰坦尼克号, why is it a 号 at the end?
From What does &quot;青年文明号&quot; mean?
号 is a common designation word for naming man-made vehicles like ships, planes and trains.
For example:
Ronald Reagan = 罗纳德·里根
USS Ronald Reagan = 罗纳德·里根号 (super-carrier named after Ronald Reagan)

泰坦尼克号 (鐵達尼號 in Cantonese) is so famous, even if you omit '号' at the end, people would still understand you are referring to the ship that sunk in 1912.
You are correct, just 泰坦尼克 (鐵達尼) would be enough for a movies title, that's why you saw it without 号.
However, if a movies is about '里根, the super-carrier', it is better to refer it as 里根号 to avoid confusion. (people would think the story is about 'the person, President Ronald Reagan' without the 号 at the end.)