There are many bridges and tunnels and a lot of extra-long slopes in the Daguan-Zhaotong Section of Nei-Kun Railway. The accumulative total length of the high slope sections is nearly 84 km, and the gradients of most sections arc above 20‰, while the maximum line design gradient is 23.5‰. In order to guarantee the safety operation of track laying during construction, the reasons of vehicles sliding down the slope and its preventive measures arc explored in this paper.