Wavelength conversion based on the cascaded sum-frequency generation and difference-frequency generation (cSFG+DFG) process in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) waveguides has the advantage of no pump occupation in the communication band, compared to the traditional cascaded second-harmonic generation (SHG) and DFG process. In cSFG+DFG-based wavelength conversion, two pump waves and a signal wave are required. The input pulse train, as a carrier of information, can be applied to either the signal or one of the two pump waves. Though the converted wave is in the form of pulses, its temporal and spectral characteristics are dependent on the arrangement of the pulsed wave. In this work, we target the wavelength conversion of 40-GHz picosecond pulse trains, and systematically analyze the temporal and spectral characteristics of the pump, signal, converted, and sum-frequency waves during their propagations in PPLN waveguides by using numerical simulation. In particular, pulse shapes, optical spectra, phases and conversion efficiency are compared when the picosecond pulse train is used as the signal and pump waves, respectively.