This research aims to investigate the pressure transients, flow separation and slipstream velocities around a freight train passing through a tunnel. The methodology consists of moving model-scale experiments at the TRAIN Rig and numerical simulations using unsteady RANS combined with the sliding mesh technique. The 1/25th scaled model represents a Class 66 locomotive connected to container wagons, entering a circular tunnel at 33.5m/s with a blockage ratio of 0.202. The effects of loading configuration, nose roundness, train length and speed are examined and the results are synthesized with 1D analytical modelling for further analysis. For the first time, it is shown that for partially loaded trains, the maximum pressure rise inside the tunnel can occur after the initial compression wave. This is attributed to the generation of low energy waves in the gaps between containers. Independent of the loading configuration, the blunt nose of the Class 66 locomotive produces a single part pressure gradient for the initial compression wave, contrary to the two-gradient rise caused by rounded noses. This pressure rise is defined by the large separation bubble around the blunt nose, which reduces the effective area and increases the blockage ratio. As the train enters the confined space of the tunnel, the separation length reduces by 31% at the sides and 32% at the roof, compared to open air. Then, its size remains unchanged throughout the tunnel with maximum lengths observed at the mid-vertical and mid-lateral positions. Separation affects the slipstream velocities which are also maximum at these locations. Velocities change with time as they are affected by the pressure waves and the position of the train along the tunnel length.