This cat sounds like it is what I call "scarel", not feral.. She just needs some time and one-on-one contact. I have socialized too many cats to even number, so I have alot of experience with these type. I agree that humanely trapping her is best. Get her spayed and combo tested and vaccinated. Then bring her home and put her in a small room or bathroom. Feed her well and attempt to play with her once daily with a feather toy or lazer light to get her attention. If she is young especially, then she will respond to the attention very well. Don't force her to go too fast, and if she lets you know she wants to be left alone, then show her the respect she is asking for and leave her alone.. She sounds like she is timid but not really afraid. Taming her may take a day or a month, or in some cases even longer.. But she will be worth it! If you need more advice on socializing her, please feel free to email me privately or check out my website at www.shadowcats.net for more info. Also check out this link for taming advice: http://www.alleycat.org/pdf/taming.pdf.

Good luck!