What does the number 3500 stand for on my Trek bike? I know that the higher the number the better the bike. My dad has a 4200, so I was wondering what kind of improvments are made to make his bike better? And also what does the number 3500 stand for on my bike?
Over the past several years, Trek has been phasing out the __00 line of bikes. 

The higher-numbered bikes are going to come with better components.

They've been basically giving non-number names (X-caliber, Marlin) and larger wheels to those bikes from the top down. All that's left at this point is the rim-brake equipped 3500. I suspect it will disappear shortly.