What does the number 3500 stand for on my Trek 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.