You ARE kidding, right?
With all the “tzuris” going on, with drugs, alcoholism, abuse, fraud, chilul Hashem, and going off the derech, you don’t REALLY mean to attack the ehrlich if time-challenged girl davening on a train?!
I mean, as you said, she’s isn’t obligated to daven at all, so what’s the difference where she does it? And why the tznius issue at all- that’s a man’s concern, isn’t it? What, she might have bad thoughts from seeing a woman with too little clothing on?
Yeshiva guy, get a life, find a real issue to work on. There are (sadly!) a lot of them out there!