Margins, service space and the structure gauge

The kinematic envelope marks how far a moving vehicle can reach. A structure placed exactly at its edge would be touched by the first vehicle whose movements happened to combine at their worst, or by the first error nobody had foreseen. Beyond the envelope, therefore, every railway requires a safety margin, a strip of space that structures and other trains must not enter. Beyond that, most railways require service space for their own operation and maintenance: room for overhead wiring masts, track machines, walkways, drainage and road vehicles, and height for future track lifts and electrification. The structure gauge combines all of these into the outline structures must stay outside.

This chapter explains the safety margin, reads the lateral and vertical dimensions of a structure gauge, using the NSW Structure Gauge 1994 as an example, covers the protection of piers near the track, and sets the running case’s pier positions and deck height.

The safety margin

ARTC’s Transit Space Standards, BDS 11, required 200 mm of safety margin outside the kinematic envelope before any fixed structure, and the same 200 mm between the envelopes of vehicles on neighbouring tracks. The margin is not spare room. It protects against the combinations and errors the envelope does not include: a track displaced further than its tolerance, a vehicle with a defect, a structure that has moved, a survey error. It is the only part of the clearance that exists purely for safety, which is why a reduction below it needs formal approval at a senior level, with conditions and records, as Chapter 9 explains, and why a designer never counts it as room that can be borrowed to make a difficult structure fit.

The commentary to the NSW Transit Space Handbook, BDS 18, explains the history: earlier NSW structure gauges, up to that of 1957, were based on a clear space of 450 mm, or 18 inches, between the train and a fixed structure, which for wide electric stock gave 2,060 mm, or 6 feet 9 inches, from the track centreline to a structure. When the 1994 structure gauge was derived analytically, adding the 200 mm margin to the kinematic envelope of the stock in use, the rounded-up minimum came back to the same 2,060 mm, which the commentary noted had a long record of acceptable risk.

Figure 1Minimum lateral clearance from track centreline to a structure in NSW structure gauges, straight track

Up to 1957, from a 450 mm clear space2,060 mm

1987 structure gauge2,135 mm

Structure Gauge 19942,060 mm

Lateral structure gauge dimensions

A structure gauge sets a minimum lateral distance from the track centreline for safe passage, and larger distances where service requirements apply. ARTC’s Structure Gauge 1994, BDS 12, gave a minimum lateral dimension of 2,060 mm on straight track for safe passage of the approved stock and loads, to be increased on curves, and larger dimensions for service requirements, applied when the corridor strategy called for them.

Figure 2Selected lateral service dimensions in Structure Gauge 1994, main lines and crossing loops

	Applied to (summarised)	Dimension from track centreline
Safe passage, straight track	minimum, increased on curves	2,060 mm
Masts and temporary works	overhead wiring and signal bridge masts; temporary construction beside a track	3,000 mm
Between tracks	piers, columns and deflection walls between tracks	3,500 mm
Bridge substructures	bridge substructures and deflection walls, and other listed structures, non-electrified, no road access	4,300 mm
With road access	structures and cuttings beside electrified tracks where road access is required	6,200 mm

The service dimensions are much larger than the safe-passage minimum. They provide room for maintenance vehicles to pass a bridge pier, for overhead wiring masts to be placed, for a road beside the track, or for workers to stand clear. BDS 12 also gave larger minimums beside sidings, where shunters work on the ground and doors may open, of 2,500 mm to 3,700 mm depending on the structure, increased near curves and turnouts.

Vertical structure gauge dimensions

Vertically, the structure gauge must clear the tallest approved outline, its rise on canted track, its bounce, the future track lift, and the safety margin, plus any overhead wiring. BDS 12 gave a minimum vertical dimension of 4,670 mm on straight track for narrow and non-electric stock, which included a 400 mm margin above the approved outline, itself including the 150 mm allowance for resurfacing, and required an increase on superelevated track. For electrified tracks it gave larger values, such as 5,650 mm for DC electrified track with the wiring attached to the structure and 5,900 mm for AC electrified track with the wiring attached.

Those values were for the rolling stock and electrification of the NSW network as it stood in 2005. Corridors that are built or upgraded for double-stacked containers need far more height, and the infrastructure manager specifies it for each corridor.

Piers near the track

A structure close to the track can be struck by a derailed train. ARTC’s Structures Design Standards for NSW, BDS 06, required bridge piers and columns within 20 m of a main line to be protected against damage from a derailed train, the main concern being to prevent the structure collapsing onto the train. Piers, columns and deflection walls up to 10 m from the track centreline were to be designed to the bridge design code, and those between 10 m and 20 m to withstand a horizontal ultimate load of 1,000 kN applied in any direction 2.0 m above rail level. It also did not allow piers between tracks except on platforms without specific approval. The clearance to a pier and its design against impact are therefore linked: moving a pier further from the track can reduce the impact design it needs, and the cost of a wider span can be partly repaid by a lighter pier, a trade the bridge designer weighs early, before the span arrangement is fixed.

What the tables already include

Structure gauge tables save work, but only if their assumptions are understood. BDS 12 stated that an allowance for curvature and cant was included in its listed service dimensions, but its minimum safe-passage dimension of 2,060 mm applied to straight track and had to be increased on curves, and its siding dimensions had to be increased near curves and turnouts. Its vertical dimensions included the 150 mm resurfacing allowance but had to be increased on superelevated track. A designer who takes a number from a table without reading its conditions can be wrong in either direction: adding a curve allowance twice, or not at all.

Temporary works

Construction beside a working railway brings temporary structures close to the track: scaffolds, formwork, cranes, hoardings, stockpiles and site fences. They are as capable of being struck as permanent structures, and more likely to move. BDS 12 gave a service dimension of 3,000 mm for temporary construction works beside a track on main lines and crossing loops. Temporary works need their own clearance checks, and their positions are inspected during the works, because a scaffold that creeps towards the track or a stockpile that slumps can become an infringement overnight. The running case’s contractor marked the clearance line on the ground beside the track with paint and pegs, and inspected it daily during the bridge’s construction.

Small things beside the track

Signs, cable troughs, lineside cabinets, fence posts, lighting columns and drainage pits are small structures, but they are structures. Each must clear the transit space at its height, and ground-level items must also clear the outline’s lower parts and the space track machines need to work. Cable routes and pits beside the track are a frequent cause of service corridor problems, because they are installed by people outside the track and civil disciplines and are often not drawn on clearance plans.

Overhead wiring

On electrified lines, the overhead wiring and its supports occupy part of the vertical and lateral space, and they need their own electrical clearances from structures, which are set by the electrification standards rather than the structure gauge alone. ARTC’s code of practice for SA, WA and Vic referred clearances between rolling stock and energised overhead equipment to its electrification provisions and the owner’s practices. Where a line may be electrified in future, as in the running case, the structure gauge must reserve the height and lateral space now, because adding them to a finished bridge is extremely costly.

Guard rails and derailment

Where a structure is especially vulnerable to a derailed train, a guard rail may be laid inside the running rail to keep derailed wheels close to the track. ARTC’s code of practice for SA, WA and Vic noted that where a guard rail is fitted to protect a structure, the structure outline should be widened to accommodate derailed rolling stock running against the guard rail. A derailed vehicle sits lower and further over than a running one, so the space it needs is different from the kinematic envelope, and the clearance design accounts for it.

Putting the margin and service space together

For any structure, the required lateral position is the larger of two distances: the kinematic envelope plus the safety margin, at the structure’s height and with the curve and cant at its location, and the service dimension required by the corridor for that kind of structure. On straight track, the service dimension usually governs; on sharp, canted curves, the envelope plus margin can approach or exceed a smaller service dimension, and both must be checked.

Figure 3Setting the inside pier under the running case's overbridge

pier offset = larger of (kinematic envelope + safety margin) and (required service dimension)

Kinematic envelope at 4,000 mm height

about 1,957 mm from centreline

Safety margin

200 mm

Service dimension specified

4,300 mm

1. envelope plus margin = 1,957 + 200 ≈ 2,157 mm
2. the service dimension of 4,300 mm is larger, so it governs
3. the pier face is set at least 4,300 mm from the track centreline, leaving over 2 m beyond the safety requirement

The deck height

For the deck, the consultancy worked to a vertical clearance the infrastructure manager specified for the corridor, which allowed for future double-stacked containers and electrification, and checked it against the tallest vehicle. A container wagon with a load 6,100 mm high, its top outer corner 1,220 mm from the centreline, rises about 79 mm at that corner on the 60 mm cant, by rotation about the low rail. Adding 50 mm of bounce, the 150 mm future track lift and the 200 mm margin gives about 6,580 mm from the design rail level, within the 6,700 mm the infrastructure manager specified in the case. This is an illustration; real double-stack and electrification clearances come from the corridor’s requirements.

Service space can be negotiated; the margin cannot

Service dimensions serve the railway’s operation, and where a site is genuinely constrained, an infrastructure manager may agree to a reduced service dimension, with conditions such as a different maintenance method or no road access past the structure. The safety margin is different: it protects against harm, and reducing it is an infringement requiring the highest level of approval, with operating conditions, as Chapter 9 explains. A designer under pressure to fit a structure into a tight site negotiates service space, and never treats the margin as available.

The running case: the drawing for approval

This is an illustration. The consultancy’s clearance drawing for the overbridge showed the track’s centreline, design rail levels, 400 m curve and 60 mm cant at the bridge; the kinematic envelopes of the freight vehicle, the passenger car and the future container wagon; the 200 mm safety margin around them; the service dimensions specified for the corridor; and the piers and deck outside all of them, with their offsets and heights dimensioned. It also showed the piers’ distance from the track, which put them in the zone where BDS 06’s impact protection applied. The infrastructure manager approved the clearances with a condition that the as-built positions be surveyed and submitted.

A failure: the straight-track pier

This is an illustration. A bridge built over a curved, canted siding elsewhere had its pier set at the straight-track service dimension, without the curve allowance the standard required for sidings near curves. The pier was outside the safety envelope, but a road-rail maintenance vehicle, which needed the full service space to pass, could not get past it on the curve.

The service dimension had been read from a table without its conditions. The siding’s maintenance had to be done from the other side of the track, at extra cost every year, and the infrastructure manager’s approval process was tightened to require the curve allowance to be shown on every drawing.

Exercise: does the margin or the service space govern?

On a sharp curve, a mast’s face, at 3,000 mm above rail, is checked against a kinematic envelope that reaches 2,350 mm from the centreline at that height. The corridor requires masts to be at least 3,000 mm from the centreline, with the curve allowance already included. Using a 200 mm safety margin, where must the mast stand, and what governs? If the curve were so sharp that the envelope reached 2,850 mm, what would change?

Envelope plus margin is 2,350 + 200 = 2,550 mm, less than the 3,000 mm service dimension, so the service dimension governs and the mast stands at least 3,000 mm from the centreline.

With the envelope at 2,850 mm, envelope plus margin would be 3,050 mm, more than the service dimension, so the safety requirement would govern and the mast would have to stand at least 3,050 mm from the centreline. This is an illustration: on the sharpest curves the envelope can outgrow the service dimension, which is why both are always checked.