During the construction of the support structure for the Hazel mine building, I needed to take accurate measurements of the distances between two vertical posts. The ones at the top of the building were easy enough to do with my calipers. The photos show the one I own. I absolutely love it. Before this one, I had a "digital" one, and I eventually threw it away, because each time I took a measurement, when I returned the unit to its closed position, the digital display showed that the calipers were some distance apart from each other. The point was, I couldn't trust that tool. The mechanical one I have now is dead-accurate, and... it doesn't need a battery!
Its dial shows distances in 100ths of an inch (blue) and in 1/64" increments (white), both of which are perfect for measuring things for S-scale.
However, during the above-mentioned structure construction I needed to measure the distances between vertical columns in the interior of the building. The calipers simply do not fit in there, so I used a trick that is used by woodworkers to accurately measure interior distances in cabinets or closets, which is to line up two straight strips of wood, get them to fit tightly into the interior of the space to be measured, and then clamp or hold the boards together long enough to take a measurement of their total overall length. I did the same thing, only on a smaller scale, by using a couple of pieces of leftover strip wood. I used a pair of tweezers to hold them together. These are the kind of tweezer that stay clamped when you let go of them. I can then use my calipers to measure their overall distance.
One day while watching a video on YouTube on how to mill and machine metal (a skill that I hope to learn in the future), the presenter mentioned a "telescoping bore gauge" to measure the interior diameter of a metal tube, for example. I had never heard of these measuring devices, but immediately thought of the challenges I was having with measuring the interior dimensions on the structure I was building and how this tool might help me with that. After doing some research, I decided to order this set by Anytime Tools out of Los Angeles, California. The link I provide shows the gauges in a pouch, but I bought mine via Amazon and they came in a nice case with marked-out foam interior to keep the gauges safe.
(external link: Anytime Tools)
If you are unfamiliar with these gauges, as I was, they operate by having the two outer "arms" slide in and out of the main housing. They are pushed out by internal springs. Each arm can be independently squeezed in. The set I bought came with six different gauges, each having a minimum and maximum distance that they can measure. This set can measure anything from 5/16" to 6".
You place the gauge in between the items whose distance you want to measure, and when happy with its position, you turn the knob at the bottom of the gauge's handle to look the outer arms in place. You can then carefully remove the gauge and measure its overall width using a caliper. Loosen the knob to release the arms to their full length. One thing I learned from that video was to make sure to buy a set that has convex (bowing outward) rather than flat ends at the ends of their "arms". The fact that they are convex means that the tool can be removed from the tight space that it is measuring, without the arms being forced closer together as the tool is removed. Flat ends leave no choice but for the tool to change its arms' lengths, meaning that you may not get an accurate measurement.
The only issue I have found with these t-gauges is that for the delicate structure I was building, the larger diameter t-gauges have quite a powerful spring in them and using them to measure the interior spaces caused the model's parts to flex. Once the model became more fully built and all of the interior parts were more sturdy, it was easy enough to measure their distances with these gauges. I think they are a very useful asset to my collection of tools, because they would also facilitate measuring the interior of freight and passenger cars and locomotive bodies, and the openings of doors and windows in a structure, etc.