Use the selection tool to select a set of carriages. If using the "Entities" selection mode, every carriage selected will be added to a single train, which also allows selecting parts of one train. If using the "Trains" selection mode, every distinct train selected will be added, which does not allow selecting subsets of any single train. The mod then simulates each train accelerating through a distance selected in the "intersection size" field, noting how long the train takes to do so. This approximates the throughput of an intersection of that size, where only one train is allowed in the intersection at once. This is also converted into more useful values like item stack and fluid throughput.
The mod also calculates the maximum speed of the selection, the braking distance at that speed, and the throughput at that speed and braking distance. It also calculates the time before the locomotive with the smallest number of fuel slots will run out of fuel, with the selected fuel.