When laying track for double-tracked mainlines, spurs, or yards, where one track parallels another track, it is important to provide enough spacing between them so that equipment on the track doesn't run into each other. Basically, on straight, parallel track, the track-center-to-track-center spacing should be at least 13 feet apart. If you hold operating sessions on your layout, and you have people managing staging yards (where the occasional human hand has to lend a hand after a derailment), the NMRA suggests a minimum of 15 scale feet of spacing.
(external link: NASG Track Centers Standard)
If you have ever seen a model railroad in action, you know that our curves are substantially tighter than track curvature in the real world. This causes equipment to hang over (i.e. away from) the track. If you have two such tracks next to each other, you have to lay the track in such a manner that two passing trains don't hit each other. This largely depends on the equipment you plan to run on your layout, and how tight those curves are. The NMRA has developed a table that lists the track-center spacing for various equipment types and various radii of the curves.
I decided to make my own track-center gauge as I need one to build a coal tipple yard. I decided to make one just for parallel straight track. If I need others, they are easy enough to make. I took a 3"x1-1/2" block of solid wood (poplar in this case) and marked off where the 13-foot marks would need to be. Since center-to-center can be measured at any spot along the tracks, I made mine such that it has notches in it for a rail. These can be for either the two left-hand rails or the two right-hand rails of the tracks being measured.
If you look closely at the photo, I cut the notches for the rail outside of the marked scale 13-foot distance. This is because the spacing between two measured rails needs to be 13 feet. Because of the fact that this gauge needs to span a certain distance between two tracks, I don't know what kind of interference there might be between the tracks (scenery, ballast, etc.), so I decided to hollow-out the spacing in between. I left about a scale foot of material to either side of the 13-foot marks I made on the wood. I then carefully cut out the rail notches with a handsaw. The kerf of the handsaw blade was skinnier than the code 100 rail heads (the rail I currently use), so I had to carefully widen the notch by cutting the outside of the kerf again with the handsaw. The middle clearance is 1/2" tall. That I did with the tablesaw, simply nibbling away at the material (other tools can be used as well).