Frequency conversion of ultrafast lasers is fundamentally constrained by the trade-off between pulse energy and repetition rate. Here, we demonstrate a frequency conversion mechanism in which molecular coherence accumulates across ultrafast pulse trains in gas-filled hollow-core fibers. Unlike conventional nonlinear interactions initiated by individual high-energy pulses, this mechanism relies on the collective buildup of coherent molecular oscillations driven by successive pulses. Using this approach, we achieve Raman frequency conversion at repetition rates up to 3 GHz with nanojoule pulse energies. The results demonstrate that accumulated molecular coherence can govern nonlinear frequency conversion, revealing a domain of light-matter interaction whose physical limits remain largely unexplored.