un trait/une ligne

[alan gould]

I know "ligne" as "line" but I see "tirer un trait" for "draw a line" so I'm wondering if you use "trait" when the line is a designation (on a piece of paper, a sign, etc.) whereas "ligne" is used for something tangible -- a line of people, a train line

merci d'avance 

[Vedas]

Yes a "trait" can be drawn on a piece of paper but it tells nothing of its orientation even if most of the time it means a straight line in order to separate two things or to underline something. Mind you there is the idiomatic expression "tirer/faire un trait sur" which means to cancel/ to forget (wilfully)/ come to terms with....and is different from "to draw a line" (set a limit).

We'd rather say une rangée/ une file de gens

"ligne" can also be the shape of an object, the figure, the outline...