What are the disadvantages of building railways in mountainous areas?

Building a railway is different from building a highway. Because the train is very long and carries a lot of goods, the requirements for roads are much higher than those for roads.

For example, it is necessary to ensure the firmness of the subgrade and prevent it from easily deforming and sinking, which will make it impossible to drive.

For mountainous sections, the railway can't cross sharp bends, so it can only dig tunnels or build difficult bridges, which also increases the construction cost of the railway.