A semi-empirical MNDO method has been used to study soliton behaviours in trans-polyacetylene. The optimized structure shows a soliton lattice. The vibrational modes for the lattice structure have been calculated. One of the C-C stretching modes is a soliton lattice sliding mode. In putting some initial kinetic energy into this mode, molecular dynamics simulations have been carried out. The charged soliton lattice can slide along the polymer chain. On applying an electric field, the automatic sliding of the soliton lattice has been observed. The sliding motion couples strongly with one of the C-C stretching modes. This coupling is found to help to stabilize the lattice sliding speed.