I think it should be "underbridge" as one word, like "underpass". "Overbridge" is another related word.

VIJAYAWADA: The proposed underbridge near Dr. K. L. Rao Nagar railway station at Madhura Nagar in the city seems to have hit a roadblock.

The Vijayawada Municipal Corporation (VMC) has to fund the project and the South Central Railway (SCR) will execute the work.

Project cost

The estimate of the total project cost for the underbridge is pegged at Rs 1.20 crores. In concurrence with the requirement, the VMC has deposited Rs. 50 lakhs with the SCR. But second thoughts among the top echelons of the VMC put the execution of the works on hold.

Usually, road underbridges or overbridges are constructed by the railways on a cost-sharing basis. But, the proposed underbridge will have to be fully funded by the VMC. The reason: the multiple of the total number of trains moving on the track and the number of vehicles passing through the road on a day is not more than one lakh. If the multiple exceeds one lakh, the railways would have participated in the funding.

The VMC has come forward to fund the underbridge. As per the project report, only light vehicles can pass through the underbridge. However, the VMC officials have felt the need to have the bridge constructed in such a manner that heavy vehicles could also pass through.

"If the VMC requires a bridge for heavy vehicles, it has to go for a road overbridge, which costs seven times more than the underbridge," says J.N. Jagannath, Divisional Railway Manager.
http://www.thehindu.com/2005/12/22/stories/2005122218590300....

Designed by Owen Williams, the new Underbridge 19A (SPC6), was successfully installed recently as part of a 72-hour possession. Constructed from reinforced concrete box units with reinforced concrete wing walls, parapets and robust kerbs, the new bridge replaces an existing bridge, which consisted of masonry abutments, and wing walls with a steel trough deck.

The installation of the bridge was made more difficult by the presence of Underbridge 19, a masonry arch bridge, located approximately 7 metres west of the new bridge. This called for careful consideration to the design of the excavation to prevent destabilisation of the arch, whilst maintaining a safe working space at the base of the cut around Underbridge 19A. To help the new bridge blend into its surroundings, the original masonry has been retained and will be used to clad the exposed concrete of the new bridge.

The project was delivered for Network Rail through the Midlands Structures Framework Contract with Birse Rail and makes a fourth successful bridge replacement within 2 years.
http://www.railwaypeople.com/rail-projects/long-eaton-underb...