The acoustic performance of a nearly-enclosed prototype meant to attenuate urban rail transit (URT) noise was measured in situ on an existing line. The prototype is constructed on a viaduct on Line 1, Ningbo, China, with a 2-meter-wide opening at the top. A series of measurements while trains passed by were carried out with twelve receiver positions at the site without and with the prototype. Train speeds were measured using a piezoelectric acceleration sensor located on a rail foot. By correcting pass-by levels for train speed, the measured results show that the nearly-enclosed prototype provides an attenuation with a maximum of 17 dB(A), and a minimum of 6 dB(A). One-third octave band spectrum analysis shows that the insertion loss specific to rolling noise yields on average more than 15 dB. Furthermore, comparisons between measured and predicted results by using 2.5-D boundary element method (2.5-D BEM) show that both of the employed PC and PMMA sheets could not provide sufficient transmission loss for the prototype in a real situation. Nevertheless, the nearly-enclosed prototype is effective against rolling noise for the URT system.