The analysis is based on the method of characteristics, making it suitable for the simulation of pressure fluctuations induced by sudden events such as trains crossing tunnel portals, shafts, cross-passages and other trains. Slower events typical of accelerating and decelerating in underground railways are handled with equal ease.

The analysis is one-dimensional. This has enormous benefits in terms of simplicity of use, robustness of the code and high computational speed, thereby making the program ideally suited for engineering design. It also has considerable benefits for the accuracy of the program predictions.

The main disadvantage of 1-D analyses is obvious, but neverthess merits comment. It is that they give no information whatsoever about three-dimensional effects. Instead, the flow state is assumed to be uniform in each cross-section. This makes it impossible for any 1-D analysis to represent details of phenomena such as flow separations at junctions or the stratification of hot smoke over colder air. One-dimensional analyses attempt to take account of 1-D consequences of such phenomena. Inevitably, they must do so in approximate manners that are most effective when used by experienced analysts who understand the implications of the simplifications.

ThermoTun offers two independent theoretical models of tunnel air flows. For historical reasons, these are designated Mode-5 and Mode-6. In Mode-6, it solves the full equations of continuity, momentum and enery. In Mode-5, it solves a simplified set of equations in which the energy equation is replaced by an assumption of homentropic flow.

In both modes, trains are regarded as a series of individual coaches, each of uniform cross-sectional area. Local loss coefficients can be specified at the front and rear of coaches to allow for flow separations. In the case of sealed trains, pressure histories inside coaches are calculated from pressure histories outside coaches, allowing for both leakage and structural compressibility.