The subject of this article is the pressure in the main reservoir of the locomotive. The purpose of the study is to obtain and analyze unique data on pressure reduction in the main reservoir of the locomotive. When carrying out train tests developed by the author of the train brake diagnostic system, pressure changes in the main reservoir of the locomotive were obtained depending on the driver's control effects in real time. This was achieved by installing a sensor measuring the pressure in the nutrient line of the locomotive and processing data using the software and hardware complex diagnostics system for the brake network of the train. In work methods of the mathematical analysis, a method of experiment, an analytical method are applied. As a result of the work, pressure reduction curves are described in accordance with the processes occurring in the main tanks of the locomotive during train operation. An explanation of the flow of pneumatic processes in accordance with the specifics of the braking equipment of the rolling stock of railways. An experiment was carried out to measure the density of the brake network of a train by a machinist and a system for diagnosing the brake network of a train, and the convergence of results was presented. Also, quantitative indicators of the measurement rate by the standard and proposed method are determined. The research results were applied to improve the methods and means of diagnosing the brake network of a train, modeling pneumatic processes of the train's brake system. Pressure reduction curves have their own gas-dynamic nature, which allows to determine the mode of operation of the braking system, as well as diagnose faults in the brake network of the train, such as shutting off end valves, leaks along the way.