I just wear cargo shorts on long road/paved rail trail trips (usually 100+ miles). I just buy 'em cheap, and they're extremely handy because of the pockets. What I really like about them are the pockets that button so I can carry my wallet and keys at easy access without fear of them falling out while riding.
I have no idea why someone would even think tight clothing would improve circulation, that's ridiculous and would cause me not to give that person's advice in other areas much credit.
What kind of rail trail? They vary from single track rough to really wide paved paths.