修建京张铁路，最困难的是从南口到青龙桥这一段。

主要是山势陡峭坡度高，还需要修很长的隧道。2作为总工程师，詹天佑每天骑着毛驴，亲自与工程技术人员去实地进行勘测。无论是骄阳似火，或是狂风大作，还是暴雨如注，他总是身背标杆和测量仪，奔波在崎岖的山间小路上。

The most difficult section was from Nankou to Qinglong Bridge. The main difficulties were the steep gradients and the long tunnels required. As chief engineer， Zhan rode a donkey and went in person every day with engineering staff to survey the sites. In scorching sun， gales and storms， he would traverse the rugged mountain paths， a surveyor’s pole and a gauge on his back.

詹天佑带领工程技术人员，历尽艰险前后勘测了三条线路。

Through all kinds of hardships， Zhan Tianyou led the engineering and technical staff in surveying three routes.

一条是关沟线——这条线路从南口至岔道城，长22公里，经过居庸关、青龙桥、八达岭，尽是悬崖峭壁，坡陡路险，工程量大，且运输量有限3。

Route one was the Guangou Route extending from Nankou to Cha-daocheng over a length of 22 kilometers， crossing the Juyongguan Pass， Qinglong Bridge， and the Badaling Ridge. The precipitous cliffs along the route meant heavy construction work and limited freight capacity on the line.

第二条线路是热河线——从青龙桥绕过八达岭，转向东北方向，经十三陵到延庆。这条线路虽然长出15公里，但坡度较平缓，也无需开凿八达岭那样长的隧道，但要专门修一条运输材料的路，费时费钱。

Route two was the Rehe Route extending from Qinglong Bridge， bypassing Badaling Ridge， turning to the northeast， and passing the Ming Tombs to reach Yanqing. Although this route was 15 kilometers longer， its slopes were relatively gentle and it did not require a tunnel as long as Badaling. But a road designed for transporting materials would have to be built， costing time and money.

第三条路就是现在修成的丰沙线——从西直门往西，绕过石景山，经三家店，到沙城附近出山到张家口。