A "clearance gauge" is a handy tool to have as well. These make sure that nothing overhangs onto the track area to interfere with the running abilities of locomotives and cars. You need one of these if your track is near structures, tunnels, overpasses, etc. They might also be handy for placing trees, telephone/telegraph poles and their wires, etc. There are three clearance-gauge standards, so be sure to pick the one for the era you model (as railroad equipment got bigger in the real world, the clearance needed for tunnels, etc. had to be adjusted to fit that equipment). For S-scale, the NASG now carries clearance gauges ("classic" and "modern", as well as for Sn3). At the time I needed one of these gauges, the NASG had the afore-mentioned gauges under development, so I decided to make my own. The clearance gauge dimensions are readily available on the NASG web site (the NMRA has them for the other scales as well), so I set out to make one for myself out of 3/4" plywood. The photo shows my final build.
(external link: NASG Clearance Gauges)
One of the notched-out corners in the bottom of the gauge is supposed to be filled-in with a 45-degree corner, but I couldn't figure out how to do that with the tools I have. That 45-degree is used to determine the clearance of braces on deck girder bridges where the braces are used to hold up the side panels of the bridge. What I may do down the road is cut a 9/32" square piece of stock, and then slice it down its middle diagonally, and then glue that into the corner of one of the notches. Otherwise, it is just something I need to remember when I get around to building one of those types of bridges. The gauge is meant to be placed on top of the rail head, so there are no notches needed for the rail. In use, you do have to make sure that the gauge is properly centered on the rails to get accurate estimates, if you are checking for clearances on both sides of the track.