I'm curious what folks use to maintain consistent spacing between parallel mainlines - tool, eyeball, straightedge or ???

Thanks.

Over the 14 years I've been following the IAIS, I've gone to great effort to confine my modeling strictly to my chosen era, both to focus my efforts as well as to limit my hobby spending.  A friend has even jokingly (?) referred to me as "heartless" due to choices I've made to part with equipment that doesn't fit my timeframe.  

In one of my recent posts ( http://model-railroad-hobbyist.com/node/18971 ) I shared a track plan I was working on for a new layout I'm hoping to build after I finish tearing my current layout down. Many readers gave helpful suggestions that made the track plan much better than I had originally designed. Since then I've made a few minor tweaks, but nothing real significant. 

Haleiwa Hotel

I am really excited about adding a hotel to the branch and found some great pictures for inspiration. The bad news is that O scale is so damned big. The image below might better be on the backdrop OR I just attach the big lanai to the photo at a slightly smaller scale (1:64?) with people inside to give a 3-D effect.