We demonstrate the evolution of picosecond pulses in silicon nanowire waveguides by sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. Due to the unambiguous temporal direction and ultrahigh sensitivity of the SFG-XFROG, which enable observation of the pulse accelerations, the captured pulses' temporal and spectral characteristics showed remarkable agreement with NLSE predictions. The temporal intensity redistribution of the pulses through the silicon nanowire waveguide for various input pulse energies is analyzed experimentally and numerically to demonstrate the nonlinear contributions of self-phase modulation, two-photon absorption, and free carriers. It indicates that free carrier absorption dominates the pulse acceleration. The model for pulse evolution during propagation through arbitrary lengths of silicon nanowire waveguides is established by NLSE, in support of chip-scale optical interconnects and signal processing.