Jairus
Author of FFVII: Lifestream & FFVII: Reflections
Well it doesn't have to be anything on that level, just something other than her usual outfit that isn't some guy's uniform. Biggs and Wedge dressed in normal clothes for their disguises, so why couldn't she? Just a chance to have a bit of what Aerith and Tifa will have later and not be left out. Besides, people on trains like that dress in a variety of ways, so she may not stand out as much as you think she might. Plus, the remake seems to be making the trains a lot more crowded than in the OG, and most passengers on trains and planes and stuff only pay attention to themselves and who they're with and don't bother anyone else. They might cast a curious glance or two, but they don't go around chatting up random strangers just because of how they're dressed. And an alluring (even modestly so) disguise could still work because of the very fact that it's so different from her normal look that no one would be expecting her to be in it.