A new method for ultrashort optical pulse train generation from a cw signal in the normal dispersion regime of optical fibers is suggested. It consists of copropagating a cw signal with a modulating pulse train at a different frequency in the normal dispersion regime of an optical fiber. The cross phase modulation, imposed by the modulating pulse train on the cw signal, can induce the generation of ultrashort pulse train from the cw signal. Numerical simulations show that the method works well under realistic practical conditions.