# Model Updating of a Freight Wagon Based on Dynamic Tests under Different Loading Scenarios
## 1 Introduction  
In recent years, rail freight traffic has experienced a significant rise in the total amount of transported goods, since railway operators have employed more efficient strategies, which include, for example, increasing the axle load, length and speed of trains, making this type of transport more competitive. Moreover, the European Union has increased its multi-annual budget to achieve more efficient and sustainable transportation systems, mainly the rail transportation. For rail freight, policy objectives for shifting goods from road to rail have been translated into a series of EU legislative measures ensuring non-discriminatory access to the market and promoting interoperability and safety [1]. In the case of Portugal, the investment in freight traffic has recently been assumed as a priority, with relevant investments planned for upgrading the existing railway corridors that connect the main Portuguese ports to the Spanish border. The effects induced by freight trains on the railway infrastructure can be more severe than those caused by passenger trains. Despite circulating at lower speeds, these vehicles usually have higher axle loads, inducing relevant quasi-static and dynamic forces on the infrastructure that can potentially put the structural safety at risk. Field measurements conducted by [PERSON] et al. [2] revealed that freight trains induce higher dynamic forces on the lower ranges of frequencies in comparison to passenger trains. This behaviour is particularly critical for the railway infrastructure, especially in the case of structures with lower fundamental frequencies, such as medium-large-span bridges [3; 4]. In these structures, there is a higher probability that the excitation frequency associated with the passage of regularly spaced groups of axles will coincide with the main structural frequencies, leading to a dynamic amplification of the responses and consequently making the infrastructure more susceptible to structural damages. Aware of the potentially critical effects that these dynamic actions may impose on structures, engineers developed several powerful numerical tools capable of simulating the complex train-track dynamic interaction problem, as well as the wheel-rail contact interface [5; 6; 7]. In addition to a robust contact model, the efficiency of these numerical tools is closely related to the accuracy of both the track/structure and vehicle numerical models. Regarding these numerical models, recent studies confirm that their performance can be significantly improved using appropriate calibration strategies based on experimental data [8; 9; 10; 11; 12]. Regarding the vehicle modelling, two main approaches can be found in the literature. The first is based on multibody dynamics [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23], and the second relies on finite element method (FEM) formulations [24; 25; 26; 27; 28; 29; 30; 31]. Multibody modelling strategies consider all the vehicle components to be rigid and connected by elastic elements, typically suspensions, which are usually modelled as spring-dashpot assemblies [17]. Concentrated mass elements are positioned at the gravity centre of each vehicle component, which are then assigned with the corresponding inertial properties. Low computational costs and good accuracy in predicting the global vehicle response make these models very popular in the design process of new vehicles, as well as in running safety investigations [13; 14; 20; 32]. Alternatively, finite element models (FEM) that consider the vehicle components flexibility through the use of shell, volume and beam elements are less widespread in modelling railway vehicles. This is especially due to high computational costs and difficulties in obtaining the vehicle design details due to the confidentiality reasons of the manufacturers. However, in problems where the influence of local modes of vibration is relevant, specifically related to the bending/torsion of vehicle components, the use of these models is required.