###### Abstract  
Rolling stock manufacturers face the challenge of manufacturing lightweight high-speed trains without deteriorating comfort. One of the difficulties is to mantain or increase structural stiffness and damping as the car bodies become lighter. Leaving aside active solutions, which are expensive and generally complex to implement, increasing structural damping by means of viscoelastic patches (via Constrained Layer Damping) seems to be a viable solution which is in fact already used for acoustic insulation in automotive, aerospace and even railway applications. Although there are works in the literature that try to optimise viscoelastic panels, this work presents an experimental study with two essential contributions: (i) to analyse the influence of a broad set of design parameters such as type of the constraining layer (uniform or honeycomb), thickness of the viscoelastic layer, location, covered area and continuity between patches; and (ii) to consider absolute and specific (per unit mass) damping depending on the design scenario. To locally increase the structural damping of an existing lightweight structure without compromising its weight, partial application of thin viscoelastic and constraining layers turned out to be the best solution. To enhance structural damping from the design stages, disregarding constraining layer mass by incorporating its stiffness into the overall stiffness of the structure, full coverage with thick viscoelastic layer and a honeycomb constraining layer with a high cross-section moment of inertia turned out to be the best option, reaching modal damping ratios up to 22 times higher than structures without viscoelastic materials. os 2020 July 202015 August 20220 July 17 August 20220 July 202  
damping material when the flexural vibration takes place on the panel. The second is the constrained layer damping treatment (CLD) in which the damping material is sandwiched by two panels stiffer than the damping material. Bending of the panels translates into shear strain of the damping material, improving its performance [2,3]. Some authors [4, 5, 6, 7] apply these passive damping technologies (FLD and CLD) using viscoelastic materials to isolate vibrations in commercial vehicles, railway vehicles and aircrafts, but they generally focus on noise reduction rather than comfort-related low-frequency flexible modes. Sound absortion in these industries is being pursued by using new low-weight materials, such as electrospun nanomaterials [8,9]. Additionally, recent works have studied the reduction in dynamic stresses and accelerations when applying aluminium foam as an energy-absorbing material for the carrying structure of railway cars [10,11]. The type of viscoelastic material as well as the thickness of the damping layer have been studied [12,13]. Location and area covered of these CLD patches are also critical factors. In the automotive, aerospace or railway industries, which strive for lightweight structures, partial-coverage (as opposed to full-coverage) is a must. [PERSON] and [PERSON] [14] were among the first researchers to study modal loss factors of partially covered sandwich beams. [PERSON] et al. [15] and, with more thorough analytical studies, [PERSON] et al. [16,17], proved that higher values of modal damping can be obtained with a partially covered beam in comparison to a fully covered one. More recently, new techniques have been used for the topological optimisation of CLD structures. [PERSON] et al. [18] first used Genetic Algorithms in order to locate optimal positions for the passive damping patches in cylindrical shell structures. Later, also [PERSON] et al. [19] performed a topological optimisation based on the Method of Moving Asymptotes. In recent years, we can find several valuable analytical contributions in this field [20, 21, 22, 23, 24]. These studies encourage us to explore the performance of different configurations of CLD railway structural components from an experimental point of view.