FREIGHT trains experience their highest longitudinal forces during rapid braking and when negotiating curves with small radii. The maximum longitudinal forces handled by buffers and draw gear in these situations limit the possible overall maximum weight of these trains.
In this context, Prose was commissioned by the Italian-based freight operator Rail Traction Company (RTC) to analyse the longitudinal dynamics of heavy freight trains and address the question of whether it is possible to drive a 2200-tonne freight train safely down the Brenner Pass, which includes a 22.5‰ gradient. Trains using the Brenner corridor on the Italian side are currently limited to 1500 tonnes when travelling up-hill and 1600 tonnes when moving down-hill. Extensive multibody simulations were therefore performed of the line to see whether an increase in weight was feasible.
Photo: Lorenzo Tosatto
High longitudinal forces inevitably occur during emergency braking applications. Normally, braking is initiated at the leading part of the train. This leads to a concertina-like motion, in which the trailing wagons press against the leading wagons and the lead locomotive.
The forces in the buffers and draw gear also increase substantially when a train negotiates a curve. This is due to the relative motion of neighbouring wagons, which leads to kinematic deflections of the side buffers and the draw gear. The draw gear will be subjected to tensional forces, and the buffers on the inner rail will be pressed together.
When combined, a scenario in which the maximum forces that can occur is created leading to an emergency braking application in a curve. This raises the question of how the variations in the starting point of braking can influence the maximum forces exhibited? Will the maximum force occur if braking is initiated at the beginning of the curve, or will it occur if braking is initiated later?
Five train configurations were investigated in the original project. However, we will focus on a train formed of Eaos wagons, as its performance can be considered typical for freight trains. The Eaos freight wagon is a four-axle open type and is used by numerous European railway companies to transport coal, ore, stone, scrap metal, timber and other similar raw materials.
The train consists of a single leading locomotive, which has comparable features to a Siemens E189, 27 freight wagons and another locomotive of the same type at the rear of the train. The overall weight of the train is 2331 tonnes and it is assumed that all wagons are equipped with the same devices: category A buffers according to EN 15551:2009 and type 1 MN draw gear following EN 15566:2016. Type Y25 bogies are used in all wagons while G-Type brakes are used on all wagons and locomotives.
In cooperation with RTC, all wagons are assumed to have the same brake application time but are activated at different times due to the time delay during transmission of the pressure drop through the brake pipe. As the pressure gradient of the G-brake hardly changes between the wagons, it is assumed as constant for each wagon. The minimum time to fill 95% of the braking cylinder is 18 seconds and the maximum time is 30 seconds. The brake forces increase linear up to a maximum value, which is determined by an analytical approach that considers the “P”-Brake definition as it is understood in real-world tests.
The track layout consists of a straight section which leads into a 280m curve via a transition curve. In addition, the slope is assumed to be 22.5‰. Initially the train runs at a velocity of 75km/h with both locomotives braking using electrodynamic brakes (ED-brakes). When emergency braking starts, the pneumatic brake application is initiated on the front locomotive and both ED-brakes are shut down. The wagons brake with a specific time delay.
In order to identify the maximum forces at play, the starting point for when the driver makes an emergency brake application varies.
The objective of the test from a safety point of view is to avoid damage to the vehicles. The following values are considered as respective limit values during the emergency brake application through the curve:
- Draw gear: The limit value for the screw coupling of 180kN for 106 cycles of loading, according to EN 15566, is chosen. The value of 180kN is a combination of the test peak-to-peak value of 170kN and a lower load, which should not be lower than 10kN and not higher than 50kN. In this case, the more sensitive lower limit is chosen to assess the results. However, when it comes to comparison with a specific case, the limit which applies to the screw couplings installed in the vehicles must be identified. In addition, one must check whether the events in which this limit is exceeded occur frequently or not and whether they impact the life-cycle of the asset.
- Buffer: The limit value of the buffer force of 250kN for 3x105 cycles of loading according to the force F6 in EN 15551 is chosen. Again, a buffer force limit value number 5 of 250kN is applied during the life-cycle tests. In the test, the force increases from 50kN to 250kN. Nevertheless, the test is carried out at the customer’s request, and it is recommended that this value is maintained. However, it is necessary to consider whether the buffers that are fitted to the vehicles were designed to meet this requirement or not.
To calculate a numerical model and provide a simulation, Prose used a multibody simulation system (MBS) software, Simpack. A sketch of a typical model can be found in Figure 1. The number of wagons leads to high calculation efforts in the multibody simulation meaning that the level of detail in the simulation models is graded with respect to the requirements of the investigation.
In the first step, pre-simulations are performed to determine the curve resistance of the specific freight wagons and the locomotives. Therefore, 3D models including rail-wheel contact are used.
In the second step, the position of brake initiation that leads to the highest forces is determined. For this purpose, a so-called 1D model of the whole train is used in which the curve negotiation is considered using the results of step 1. In this context, the term 1D refers to the longitudinal degree of freedom of the bogies and wheelsets along the horizontally and vertically curved track.
Figure 2 shows the maximum buffer and draw gear forces as a function of the position of the front bogie of the leading locomotive, Bo1, when braking is initiated. Here, s = 0 m describes the case when the first bogie Bo1 is reaching the full curve.
As expected, it shows that the forces are higher in the right buffer as the train is negotiating a right curve. For both sides (left and right) the maximum force inititally increases following the increasing position of brake initiation. For the right buffer, a maximum force of 133.4kN is reached for s = 192m. For the left buffer, a maximum force of 108.5kN is reached for s = 82.4m. After the peak, the maximum forces decrease in both cases and reach a plateau for s > 470m.
Figure 2 also shows that the trend of the draw gear force consists of two plateaus with roughly constant values, where a higher force is reached for braking positions when s > 470m. The maximum draw gear force of 164.3kN is reached for s = 695.4m.
If braking is initiated while the freight train negotiates a curve, the buffer forces will be a superposition of the kinematic deflections of the buffers and the compressive forces induced by the pushing of the trailing vehicles against the leading vehicles, where braking has already started.
In cases where braking is initiated before the maximum position is reached, part of the train will decelerate when entering the curve and the buffers will have dissipated some of the energy of the concertina-like movement that follows the brake application. If braking is started after the maximum position is reached, almost the whole train has entered the curve. As a consequence, the buffers are already pushed together by the kinematic deflection. One can argue that in this situation, the superposition of the braking will not increase the forces that much, because the level of force of the buffers is already high, and the acceleration induced by braking will be lower than in the maximum case. Hence, the maximum force is reached between these two scenarios, a situation where the superposition of both effects leads to the highest forces.
Considering the dimensions of the Eaos train and the length of the transition curve, one can conclude that the draw gear forces are highest if braking is started after or while the rear locomotive has entered the curve. As the braking position is of no influence on the maximum draw gear force for s > 470m, it can be concluded, that the maximum draw gear force is given by the superposition of the ED brake of the rear locomotive and the kinematic deflection of the draw gear when the rear locomotive enters the curve. The other way around, the maximum force is decreased if braking starts before the rear locomotive has entered the curve.
For the train of Eaos wagons, the maximum values in the buffers (133.4kN) and draw gear (164.3kN) do not exceed the limit values of 250kN for the buffers and 180kN for the draw gear.