Last reviewed 30 Sept 2026 · 6 min read
Railway bridges — what is different
A railway bridge carries heavier, repeated and more concentrated loads than a highway bridge, on a track that needs tight geometric tolerance: a few millimetres of deflection or twist can be a safety problem. Design is done to Indian Railways Bridge Rules and the associated codes for concrete, steel and substructure, with the standard track and loading.
Bridges are classified by their waterway length:
| Category | Description |
|---|---|
| Minor bridge | Total waterway less than about 18 m |
| Major bridge | Total waterway of about 18 m or more, or a span of about 12 m or more |
| Important bridge | A major bridge of special importance, e.g. long-span or over a major river |
The categories decide inspection and maintenance frequency and who has authority for decisions.
Types of railway bridge decks
| Deck | Description |
|---|---|
| Ballasted deck | Track on ballast laid on a concrete slab or steel troughs; smooth ride, easy track maintenance, distributes load; heavier |
| Open deck | Sleepers directly on girders (girder bridge); light; needs bridge timbers or steel sleepers and separate maintenance; noisier |
| Ballastless track | Slab track; modern high-speed and metro |
Through girder — the track runs between two main girders (low construction depth). Deck girder — the track runs on top.
Standard spans
Indian Railways uses standardised spans to speed design and construction: RCC slabs and boxes for short spans, PSC girders (T-, I- or box), steel plate girders for spans up to about 60–70 m, and steel trusses for longer ones. Standardisation enables repetition of casting and design.
Loading
The bridge is designed for the standard locomotive load with dynamic augment, longitudinal (tractive effort and braking), lateral (nosing and wind) and centrifugal forces on curved tracks, temperature and — for steel — the effects of rail-structure interaction (long welded rails on a bridge transfer forces through the fastenings). The fatigue and derailment cases are specifically checked for the deck and the ballast retainers.
Deflection limits for railway bridges are stricter than for highway bridges, to protect the track geometry; use the value in the current Bridge Rules rather than a remembered figure.
Road-over-bridge (ROB) and road-under-bridge (RUB)
Where a road crosses a railway, a level crossing creates delay and danger. To remove it:
- ROB — Road Over Bridge: the road is carried over the railway on a bridge with approach embankments or viaducts. The clear headroom above the track (vertical clearance above rail level) and the horizontal clearances are specified by the Schedule of Dimensions. An ROB needs long approaches (the approach gradient is kept flat, typically not steeper than about 1 in 40) and land, so it costs more but is preferred on busy roads and where the railway has many lines.
- RUB — Road Under Bridge (subway): the road passes under the railway through a box or girder bridge below the tracks. It needs less land, but the vertical clearance is limited (about 3–5.5 m depending on road use), the road may need drainage (pumping) and lighting, and the bridge must be built with the railway in service.
A limited height subway (LHS) is a narrow, low-clearance RUB — typically 2.5–3 m high — for pedestrians, cycles and light vehicles at locations where a normal RUB cannot be built. It is quicker and cheaper and is the usual means of closing a level crossing on a lightly used road.
Construction of RUB / LHS under traffic
Since the railway cannot be stopped for long, box pushing is used:
- A RCC box is cast on a launching platform beside the embankment.
- Hydraulic jacks push the box through the embankment while a shield at the front cuts and excavates the soil; the track above is supported on temporary girders (girder bridge) or is monitored.
- The box is pushed a metre or so at a time in a traffic block (a planned railway closure) or between trains.
- After it reaches its final position, drainage, wingwalls and the road surface are completed.
Cut-and-cover with girders supporting the tracks and pre-cast portal frames are alternatives; pipe roofing may be used ahead of the box.