Single girder bridge is only one track, so only to find a point at both ends of each elevation can be the same.

单主梁桥架上只有一个轨道，所以只找出两端各一点的标高相同即可。

youdao

As we cross the snaking, single-track bridge there is a glimpse of Loch Shiel, and of the monument that marks the spot where Bonnie Prince Charlie first assembled his Jacobite army that nearly did so well for the Scots.

BBC: Full steam ahead