underbridge

under- +‎ bridge

underbridge (plural underbridges)

1. (British) A bridge that allows traffic to pass under a road, river, railway etc.
   • 1979 August, Michael Harris, “A line for all reasons: the North Yorkshire Moors Railway”, in Railway World, page 412:

     As to underbridges, the policy is to wait until the small underline bridges become due for renewal when they will be replaced by prestressed concrete structures.

   • 2020 December 2, “Network News: Green light for NR to close crossings”, in Rail, page 23:

     Additionally, NR has been given permission to alter highway rights at the underbridge at Littleport station.

2. The underneath of a bridge.

Where a bridge takes one form of transport over another it is both an underbridge and an overbridge, depending on the reference level. For example, where a road passes above a railway, the bridge is an overbridge from the point of view of the railway, and an underbridge from the point of view of the road.

• overbridge

• “underbridge”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.