I think u must get the wrong information. There isn't any train from Chengdu to Lhasa , there isn't a railway from Chengdu to Lhasa. No railways reach Lhasa. But there are three highway to Lhasa, the Qingzhang highway, The Chuanzang Highway and the Tianzang highway. And there isn't any directly bus from Chengdu to Lhasa.
You can hire a jeep from Lhasa to Zhangmu, the border to Nepal. and then Pass the border, then hire another jeep from Zhangmu to Kodari, the customs of Nepal. then from Kodari take a jeep to Kathmandu.