## 4 Discussion  
To improve the efficiency of the transportation process, the design of a detachable module for the transportation of long cargoes is proposed (Figure 1). The choice of profiles for the detachable module is based on the resistance moments of its components. For this purpose, the relevant calculations are carried out in LIRA-CAD. In this case, the detachable module is considered a rod system (Figure 3). Mathematical modeling is carried out to determine the longitudinal loads acting on the detachable module in operation when it is placed on a flat car. The most unfavorable loading condition of the flat car, i.e., the shunting impact, is taken into account. In accordance with regulatory document [25], a longitudinal force of 3.5 MN is applied to the rear stop of the coupler. The calculations show that the maximum acceleration acting on the flat car loaded with two detachable modules is about 37 m/s\({}^{2}\) (Figure 6). This acceleration value, as a component of the dynamic load, is taken into account in further calculations of the detachable module. Based on the calculations, a structural channel reinforced with a vertical sheet is chosen as the profile of the detachable module (Figure 13). This decision is justified by the fact that the minimum safety factor in accordance with [25] should be 1.3. Therefore, to ensure an adequate safety margin, it is proposed to reinforce the structural channel with a 5-mm vertical sheet. The strength calculation of the detachable module under asymmetric loading diagrams in operation (perception of longitudinal loads--mode I, and lateral loads--mode II) is carried out. It is found that the maximum stresses in the detachable module structure at mode I are 286.5 MPa (Figure 16a). These stresses are 7.7% lower than the permissible values. The maximum displacements in the structure of the detachable module occur in the upper belt of the end superstructure and amount to 4.1 mm (Figure 17a). The maximum stresses at mode II are 292.4 MPa (Figure 16b). It is important to note that these stresses do not exceed the permissible ones and are 5.8% lower. The maximum displacements are recorded in the central posts on the tilt side of the detachable module and amount to 6.2 mm (Figure 17b). This study also includes a modal analysis of the detachable module. Traffic safety is assessed by the first natural oscillation frequency, which, in accordance with [25], should be at least 8 Hz. The calculations show that the first natural frequency of oscillation is 20 Hz. Thus, traffic safety is ensured from the point of view of modal analysis. The limitation of this study is that the mathematical modeling of the longitudinal load of the detachable module includes a rigid fixation of the freight (zero degrees of freedom). The disadvantage of this study is that it includes only two loading diagrams of the detachable module. However, this disadvantage can be justified by the fact that the modes in question are the most unfavorable regarding the detachable module load. The advantage of this study in comparison with [8] is that the proposed design of the detachable module does not require a special maintenance and repair system and can be carried out at rail enterprises using the existing technologies. In contrast to the vehicle designs proposed in [9; 11], the detachable module can be used to transport a variety of freight, apart from long cargoes. Compared to the results of [10; 12; 13; 14], the proposed detachable module design is mobile, which increases the efficiency of its operation in contrast to flat cars. The disadvantage of the solutions proposed in [15; 16] is that they require situational adaptation of the existing wagon fleet for the transportation of long cargoes. Moreover, the introduction of detachable modules is more efficient, especially for international transportation.