Different, but connected.
When one of Britain's earliest steamship companies, the Royal Mail Steam Packet Co, was formed in the 1830s it was first proposed to call it the "Royal Mail Line of Steamships" to give the sense of a regular sailing of ships along the same track carrying mails (which subsidised the whole operation) to the Caribbean and eventually on to South America.
Unlike a line of battleships it couldn't be seen but it was a "line" of ships all the same.