The quote refered to in this question comes in scene 6, after Mitch and Blache return from their first date, which has not been successful. As Mitch talks about heading home, Blanche asks him the following question, whilst "laughing grimly":
Is that streetcar named Desire still grinding along the tracks at this hour?
What is so interesting about Blanche's character is the way that she is split between pretending to be virtuous and rejecting promiscuity, whilst underneath there are clear sexual overtones to her speech that hint at her true character and her "epic fornications." Not only does the question about the streetcar relate to Blanche's past and her reason for coming to Stella, the use of the word "grinding" hints at the sexual act in a way that Mitch is blind to. Blanche is trapped between her real nature and her desire to sleep with Mitch, but then also her need to present a virtuous and honourable face to the world in order to try and be the prim and proper Southern belle she pretends to be.