## 1 Introduction  
Higher operational efficiency, as a precondition for maintaining the leading position in the global transportation market, can be achieved through the introduction of articulated wagons on railways. A special feature of such wagons is the two-section carrying structure resting on three bogies. The sections interact through a joint assembly.  
The lower material capacity of the carrying structure of a wagon, under observance of the conditions of strength and operational reliability, can be achieved with the use of circular tubes in the carrying structure elements. Previous research enabled us to conclude that such implementation could decrease the rare weight of a wagon by 3-5% in comparison with that of a prototype wagon. However, the problem of determining the fatigue strength under operation remains unsolved. It is known that one of the most considerable loads on a wagon in operation is the dynamic load, and the main devices absorbing the kinetic energy that affects the carrying structure of a wagon are automatic couplers or buffer devices. However, they cannot fully absorb the kinetic energy under increased normative values of dynamic loads. It causes damage to the carrying structures of wagons in operation and requires off-schedule repairs. Therefore, there is a need to develop and implement new draft gear concepts for rail vehicles.  
The structural and design features of a universal long-base wagon for intermodal transportation in Europe are presented in [1], which provides the results of strength calculations for the carrying structure of a wagon. However, the study does not propose any measures to decrease the dynamic load under operation at the design stage.  
The Department for Mechanics and Applied Information of the Military University of Technology (Poland) has developed a specialized wagon with a low rotary loading platform [2]. The wagon is intended for transportation of trucks by rail. A previous study [3] presented the calculation of dynamic indices for this wagon. The factor of safety against wagon overturning was studied when the wagon entered a curve at various traffic speeds. However, the issue of improving the coupling device of the wagon to reduce its dynamic load in these works was not studied.  
Some measures to improve the carrying structure of a wagon are proposed in [4]. The study provides the results of a strength calculation for the carrying structure of a wagon realized in LIRA software. The authors considered the normative load values on a wagon in operation for the strength calculation but the study does not give refined dynamic load values on the improved wagon structure.  
The mean dynamic load on transport under a longitudinal impact is presented in [5]. The study used accelerations as dynamic load components for the experimental variable. The authors did not propose measures for improving the wagon coupler to lower the dynamic load during operation.  
The structures of the BCNHL wagons are analyzed in [6]. The authors discuss some possible ways to improve the technical and economic parameters of wagons for higher operational efficiency.  
The results of the structural finite element analysis of a freight wagon using the finite element method are presented in [7]. The research was conducted using the BOXN25 open wagon used by Indian Railways.  
The objective of this study was to demonstrate some peculiarities in computation of the dynamic load and strength of the carrying structure of an articulated wagon of circular tubes. To achieve the objective, the following tasks were set:  
* To propose techniques for lowering the dynamic load on articulated wagons in operation,
* To conduct mathematic modeling of dynamic loads on the carrying structure of a circular-tube articulated wagon, and
* To determine the basic strength parameters of the carrying structure of an articulated circular-tube wagon.