Well, I know I'm a little late to chime in here, but a 5/8 wave antenna will work better than a 1/4 wave antenna will, though granted, they are a lot taller...if you can deal with that though, go for a 5/8 wave antenna.
If you want something that's already pre-tuned and ready to roll, magnet mount and all, I'd recommend the Railcom antenna:
http://railcom.radio-dealers.com/aservd ... 4&cat_id=3
If you don't mind a bit of sweat though, you can easily purchase a Radio Shack antenna and cut the antenna to the correct length.
Course, if you wanted REALLY good reception, you could get a radio instead of a scanner, but that's a whole 'nother ballgame...