Joyce Carey's character?

I'd say the Joyce Carey romance subplot really adds to the film. It's there to counteract the deep and passionate emotions between Alec and Laura, and the film would've been too heavy without a lighter side story. I think it adds even more power to the film when Joyce Carey and Stanley Holloway are comically and light-heartedly doing their flirting, while yards away, another couple are going through complete heartbreak.

The minor romance indeed didn't culminate in anything, and it didn't need to. It served its purpose, and gave a few laughs!

What Joyce Carey's character seems to portray is the ease that she sorts out her life while Laura and Alec agonise over their affair, Myrtle Bagot has no such qualms " I packed my boxes there and then and left him" moved in with her sister for a bit then opened up a tea shop in Hythe. She shows an independence that middle class Laura could never hope to attain, she leaves her husband, becomes financially independent is still respectable and eventually runs the cafe on the station. I am sure Coward was trying to say something with this "subplot" between the working and middle class characters, has a reflection of his own sexuality perhaps.

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I believe that she invites the conductor around, after he throws the two soldiers (were they?), out of the cafe. It appears she sees a different side of him when he does that, and she likes what she sees!