Symbols can help a writer convey ideas, develop characters, establish atmospheres, etc. To what effect were symbols employed in A Streetcar Named Desire?

A significant symbol is contained within the play's title. According to Tennessee Williams, the streetcar is "the ideal metaphor for the human condition." The specific streetcar line that is referenced conveys the power of sexual desire that influences people's actions; the symbol furthers this dominant theme in the play, and its meaning is contained in the name of the streetcar line Blanche takes:

They told me to take a street-car named Desire, and transfer to one called Cemeteries, and ride six blocks and get off at—Elysian Fields!

Blanche views herself as an object of male desire and flirts aggressively and openly. Stanley learns that Blanche had been fired from her teaching position for having a physical affair with a student and evicted from a motel she had been staying at after having numerous sexual encounters there. The marriage between Stanley and Stella revolves around sexual gratification, and Stanley rapes Blanche after he learns of her unchaste past. The streetcar line is a representation of Blanche's journey in life, in which she must follow the path on which desire takes her to its end.
Alcohol also functions as a symbol, with Stanley and Blanche drinking consistently throughout the play. Stanley often becomes violent and physical while drunk; he beats Stella while intoxicated, and both himself and Blanche are drunk when he rapes her. For Stanley, alcohol represents the theme of masculinity and physicality. In Blanche's case, it represents her escape from reality, illustrating the theme of fantasy and delusion. For instance, Blanche is very intoxicated when she imagines she is hosting a high-class party, complete with hallucinations of male admirers.
Moreover, polka music functions as a personal symbol for Blanche, as she associates it with her husband's suicide; after confronting him about his homosexuality while they are dancing polka, he leaves the dance floor and promptly kills himself. Blanche and the audience hear polka music when she thinks of the circumstances of his death; this contributes to the sombre atmosphere of scenes in which the music is played and conveys Blanche's emotions.
Three additional symbols convey Blanche's urge to escape reality: bathing, darkness, and the paper lantern. Blanche seems to be constantly bathing throughout the play; this represents her attempt to cleanse herself of the unpleasant reality of the apartment, as well as the guilt associated with her past. Blanche prefers night and the dreamworld and demonstrates an aversion to light:

I can’t stand a naked light bulb, any more than I can a rude remark or a vulgar action.

Blanche would rather hide behind politeness and niceties than accept the truth of someone having an unfavorable opinion of her. She covers the bare light bulb with a paper lantern; the lantern is fragile, just as Blanche's mental state is, as well as her illusions.