Try approaching her and saying something like:
"Hey, I mean this with absolutely no strings attached, but I had to tell you, you look really good today."
And then walk away. It will totally make her day and she won't forget it or you. Then, if you commute at the same time everyday, you're sure to see her again. Even if you ignore her when you do see her, or just kind of smile in her direction, she will feel internally compelled to come and talk to you if she's attracted to you.
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