What are two reasons why railroads use a large number of movable bridges?
All time & materials are reduced to cost.
Travel time to detour the railroad around the possibility of the bridge location. It's all cost.
You might argue that the TIME saved by the shorter route will get the products to market faster.
You've probably found this already:
From Wikipedia, the free encyclopedia
A moveable bridge is a bridge that moves to allow passage for (usually) boats or barges. By making the bridge moveable it may be made low, which avoids the expense of high piers and long approaches, greatly reducing the cost of the bridge.