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- In BIMpedia
Railways look simple from the platform: two rails and some wires. Behind that sits one of the most tightly integrated pieces of infrastructure there is, with millimetre tolerances over hundreds of kilometres, several disciplines sharing a narrow corridor, and a live railway next to the work.
| System | What it is |
|---|---|
| Track | Rails, sleepers, ballast or slab, switches and crossings |
| Structures | Bridges, viaducts, tunnels, retaining walls, stations |
| Overhead line equipment (OLE) | Masts, wires and their foundations, on electrified lines |
| Signalling and telecoms | Signals, train detection, cables, equipment rooms |
| Drainage | Track drains, culverts, cesses |
Track geometry
Rail alignments have everything a road alignment has (straights, curves, transitions, grades, vertical curves) plus cant: the outer rail raised on curves. Cant changes gradually along transitions, and the rate of change is limited. Standard gauge, the distance between the rails, is 1,435 mm on most of the world's main lines.
Clearances are about the moving train
The question is never just "does this touch the track?" but "does it stay clear of the space a moving train needs?". That space is described by a gauge (a standard profile) or a kinematic envelope (the train's possible positions including sway and cant), plus electrical clearances around the overhead wires. Model those volumes and clash against them.
Possessions and sequence
Work on or near the line happens in possessions: booked windows when the line is closed, often a few hours at night. Designs are shaped by them: prefabricated bridge decks lifted in one night, foundations that can be built from the cess while trains run. 4D sequencing is often worth more on rail than anywhere else.
Locating rail assets
Railways locate assets by a line reference and a distance along it: kilometres or, on older networks, miles and chains (1 chain = 22 yards ≈ 20.12 m). IFC 4.3 adds rail elements, track parts and cant to open BIM, so a model can carry this referencing.
A new footbridge is clash-tested against the track and the platform edges. No clashes. Late in design, the OLE team checks electrical clearance and finds the deck too low over the wires on the curve, where cant tilts the train and its pantograph towards the bridge.
The coordinator asks the track and OLE teams for the kinematic envelope and the electrical clearance volumes along the bridge, and adds them to the clash matrix. The first run shows the shortfall at concept stage, and the deck is raised before anything is detailed.
A culvert under a railway is recorded at 12 miles 40 chains on its line.
- Convert that to kilometres along the line (1 mile = 1,609.344 m; 1 chain = 20.1168 m).
- A new drainage model uses metric chainage from the same origin. What chainage should it show?
- What must both records also state for the location to be unambiguous?
Key takeaways
- Track geometry adds cant (one rail higher than the other on curves) and transitions to the usual alignment.
- Clearances are checked against the space the moving train needs (gauge and kinematic envelope), not just the track.
- Work near the line happens in short possessions; sequence and logistics drive the design.
- Rail assets are located by line reference and mileage or chainage; IFC 4.3 adds rail elements and cant.