Here are three common construction mistakes that will bite you in the rear if you aren’t careful.  I’ve made all of them.  Most of the other modelers I come in contact with have made all of them.

Parallel Track Spacing

In HO scale, a parallel track spacing of two inches is the norm (one inch in N).  However, that only applies to tangents.  If you have parallel tracks on a curve you need to increase the spacing to around two and a half inches (1 1/4″ in N).  If you don’t,  longer equipment will sideswipe each other.

Rail Gaps

Just as on the prototype, our miniature rails are made of metal.  As such they will shrink and expand with temperature.  In addition, our plywood sub-roadbed changes dimensions with humidity.  If you don’t allow space at the rail joints for expansion and contraction,  at some point it’s likely you’ll walk into your basement to find some nasty buckled rail.  Allow about a 1/16 inch gap every other rail.  In addition, never, ever solder your rail joints (the exception being when you need to create longer sections of flex track for curves.  There you will need to solder the joints).

Overly Thin Electrical Feeders

With apologies to the EE’s and electricians reading this, think of your layout wiring like the water main and water feed system in your neighborhood.  If the “pipe” or wire is too small the “water” or electrons can’t flow in an unrestricted manner.  Inadequately sized feeders won’t get the current you need to the rails and will result in sluggish, brownout performance.  It can also result in overheating of the wire.  As a general rule, I use 18 gauge feeds roughly every other rail (soldering a feed to every rail is unnecessary overkill)