<p>Carbide-free bainitic steels are increasingly employed in high-performance rail systems for their exceptional wear resistance. However, achieving an optimal strength-toughness balance under hot-rolled and air-cooled conditions remains challenging due to the complex multiphase microstructures and coarse bainite formations. This study systematically compares microstructural evolution and mechanical properties of carbide-free bainitic rail steels with low-carbon (0.26 wt%) and high-carbon (0.32 wt%) compositions under Nb and Nb-Ti microalloying. Nb-Ti microalloying facilitates epitaxial NbC nucleation on pre-existing Ti(C,N) precipitates during thermal processing, thereby weakening the dislocation drag effect of solute Nb and reducing the efficacy of prior-austenite grain refinement by 53% compared to steels microalloyed with Nb alone. Conversely, single Nb microalloying synergistically refines prior-austenite grains and bainitic blocks, increasing high-angle grain boundary density and elevating impact toughness by 28%. In high-carbon systems, the strengthening effect from bainitic lath refinement is counteracted by coarsened M/A constituents. The optimal composition (0.26 wt.% C with Nb microalloying) achieves a balanced strength-toughness combination in hot-rolled air-cooled conditions through coordinated control of bainite lath thickness, refined M/A morphology, and enhanced high-angle grain boundary density.</p>