In 'A Streetcar Named Desire', what is the effect of Williams' use of plastic theatre?

Plastic theatre utilises props, sound, stage direction, and costume to present poetic truths through symbolism. It is not intended to be realistic, but symbolic. In A Streetcar Named Desire, one of the ways Williams uses plastic theatre is to represent the way the world of Stanley (as a figure of 'New America') is gradually closing in on Blanche, to devastating effect. Stanley's perpetual presence is conveyed as 'A locomotive is heard approaching outside'. Williams crafts this stage direction to foreshadow the ending of the play, as the locomotive is symbolic not only of Stanley's troublesome omnipresence in Blanche's life, but also represents the future- a future that Blanche cannot fit into because she is tethered to her past glory as a 'Southern Belle' in the 'Old South'. It is important that the locomotive is 'heard approaching', as this sound is a convention of plastic theatre. The sudden approach of the locomotive interrupts Blanche's rare moment of candour, so this loud noise symbolises the end of her age of innocence, and the beginning of her pretences. Williams evokes sympathy for Blanche through his presentation of this, as, after hearing of her guilt after her traumatic experience as 'a very young girl', audience members understand that the trajectory of her life has been determined by tragedies she is unable to overcome. Williams' intention both in writing the play and in using plastic theatre is to side with the marginalised; he wants audiences to pity Blanche despite her failings, because he (as a homosexual in 1947) related to this feeling of being an outsider. The presentation of the locomotive 'approaching' and the fact it has a presence that is so pervasive it is seen and heard, serves to further emphasise Williams' sympathy for Blanche, as its brash presence is opposed to the refinement of the 'Old South', with which Williams (like Blanche) sided.