This paper describes a scale modelling method to measure the acoustic performance of a nearly-enclosed barrier and the corresponding predictions using an existing 2.5-D Boundary Element Method (BEM) program. Preliminary investigation results show the deterioration in performance of a nearly-enclosed barrier due to resonance effects that led to high pressure levels radiating into the surroundings via the topped opening. Measurements on one-twentieth scale model of barriers, viaducts and vehicle structures were carried out outdoors under controlled conditions. The measured results show that the transmission loss of transparent panels on the top were not adequate to make the measured results as high as the predictions. A modified scale model, by coating all the surfaces with rubber, was remeasured. The measured results from the modified scale model and the numerical calculations were in good agreement each other, which indicate that the 2.5-D BEM code can provide a reliable description of the acoustic performance of a nearly-enclosed barrier. Then the program was used to investigate the barrier performance for different receiver positions in the surrounding environment. As expected, the attenuation of the nearly-enclosed barrier averaged around 15 dB in the near field and around 10 dB in the far field. The number effect of incoherent point sources on the performance is discussed as well for the study of railway traffic noise. The increased number of incoherent point sources can result in smoother and lower attenuations for the whole sound field.