You referring to the FX series bikes? Might want to repost in the Hybrid forum.

My wife has a 2.1 WSD road bike. For hers it was a different geometry frame, narrower and shorter drop handlebars, shim wedges in the shifters, and a more feminine designed saddle. More than anything else it was the *only* 47cm (Wife is 4'11") that she came even close to fitting on and 43cm was the next step down and too small. So essentially it made the difference between owning a road bike and not owning one.

As to the FX series differences probably better to ask in the hybrid forum as to the specific differences. Trek also has some info at: http://www.trekbikes.com/images/wome...df/bikefit.pdf