AbstractIn this study, field testing is conducted on two full-scale guideway reinforced concrete (RC) beams, one reinforced entirely by glass fiber–reinforced polymer (GFRP) and the other by conventional steel, under 450 passes of two-car monorail trains at various speeds and vehicle loads. The two beams were part of a 1.86-km test track traveled by trains for 4.5 months. The objective of the study was to evaluate GFRP bars as a sustainable reinforcement for transit infrastructure, particularly for monorail trains. The study showed that cracking and stiffness degradation in both beams occurred during train loading with one-loaded and one-empty cars, after 79–132 cumulative train passes. Cracks stabilized after 149 accumulative passes. Deflection in the GFRP-RC beam, measured during train passes at fully loaded cars, was 36%–47% higher than that in the steel-RC beam. After 200 accumulative passes, stiffness was reduced to approximately 43% and 57% of the original uncracked value for GFRP- and steel-RC beams, respectively. By the end of 450 passes, stiffness in the same beams appeared to have stabilized at 38%, and 50% of the uncracked value. The GFRP-RC beam satisfied all serviceability limits imposed in national codes. A two-dimensional finite-element analysis was also performed on the test beams, showing good correlations with test results of deflections and cracking patterns. Of the six analytical equations that were used to estimate the effective moment of inertia and subsequently beam’s deflection, only those by Branson and Bischoff seemed to yield acceptable predictions.