I'm wondering why is it that a 42mm chainline is considered 'better' for a track style (120mm spaced) fixed bike?

It seems to me that a 46mm chainline would be better because it allows for an undished (stronger? better looking?) rear wheel.

Or am I just stupid? Am I about to do something very wrong with my (first) build.

thanks!

42mm is industry standard. It might not be better, but as its standard theres more bits out there made to fit to it.

No reason to go with the crowd though, do whatever you feel is best. My approach to chainline has always been to put it together and if it's not right fiddle with it until it is. So far everything's worked (more through luck than judgement), but I'm sure one day I'll build something up and it'll all be on the piss and then I'll actually have to put some thought into BB lengths etc :/

I don't think its a case of better, more that it works, and its all driven by the rear hub, you'll get a different chainline with a different sized rear hub, track bikes work off a 120mm hub so you get a chainline in the 42.5 - 43.5mm, if you wanted a chainline in the 46-47mm area what size would your rear hub be? I don't know as I'm just regurgitating what I'e been told, and have come to believe is the truth, if you can get a chainline in that zone without it being a bitch to figure out/get parts for go for it.

I'm planning to use the http://www.londonfixiebike.co.uk hub which they say, with appropriate spacers will make for a dished wheel at 42mm and undished at 46mm.

My bottom bracket is a road campy with campy pista cranks. I can't remember the axle length but I've assembled and measured it at a chainline of around 47mm.

Surely bringing the chainline inwards means your q-factor is smaller?
This is more efficient and therefore faster. Faster is what you want when track racing.
"stronger and better looking" aren't really required of track wheels when ridden on a track with no bumps, as long as they take the rider input without wasting energy.

ah, so it's all about hip width

hip is wide.. it's all the beer he drinks..