Rail transit aluminium extrusion: large hollow car-body profiles and the handling that limits them

2026-10-09 18:02:23

Rail transit aluminium extrusion is the most demanding end of the whole industry. Metro, commuter and high-speed car bodies are welded from very large hollow sections — side walls, roof, floor — that are metres long and weigh 30–60 kg per metre. The press gets the headlines, but on a rail transit line it is the haul-off, cooling bed and saw that decide whether the plant actually ships tonnes per hour or just makes impressive ingots.

The large hollow profiles of a car body

A modern railway vehicle is a monocoque built from extruded aluminium. The shell is a kit of wide, thin-walled hollow sections joined by friction stir welding (FSW). The main families are:

• Side wall — the largest, often 700–900 mm wide and 30–50 kg/m.

• Roof — curved or multi-cavity, 25–40 kg/m.

• Floor / underframe — the heaviest, carrying crash and torsion load.

• Bodyside stringers — the stiffeners that FSW onto the shell.

• Door and window surrounds — precise, often anodised.

Alloys: 6005A, 6082 and 7N01

Rail bodies favour heat-treatable 6xxx and some 7xxx alloys. 6005A is the workhorse for skins and roofs thanks to good weldability and extrudability; 6082 is chosen where more strength is needed in floor and underframe; 7N01 (a Japanese-origin Al-Zn-Mg grade) appears in high-strength structures. All need tight control of Fe, Si and trace elements so the FSW seam stays sound. Our alloy guide covers the 6xxx family in detail.

Why rail needs very large presses

Extrusion force scales with the perimeter of the section, not its mass, so a wide hollow wall needs far more tonnage than its weight suggests. Car-body sections up to 900 mm wide typically run on 8,000–12,000 t presses with a large container taking a 400–600 mm billet. Below that range the die cannot be filled at a usable speed and the section centre goes short. The press is only half the story, though.

Car-body profile families and tonnage per metre

Why handling becomes the limiting factor

At 30–60 kg/m and 20–30 m long, a single length weighs 600–1,800 kg. The haul-off machine must pull it out of the die without sag or twist; the cooling bed must support it full-length while it cools from 520 °C; and the single-bar saw must cut it square for welding. On a rail line the auxiliary equipment — not the press — sets hourly output, so we oversize pull force, bed length and saw clamping accordingly.

Friction-stir-weld-ready flatness

FSW joins the shells without filler, so the mating faces must be flat and consistent or the tool loses penetration. We typically hold ≤0.2 mm flatness over the weld seam and ≤1 mm/m straightness, set by the straightener and confirmed at the saw. That flatness is what lets a bodyside weld at 1–1.5 m/min without porosity.

Quality system and traceability expectations

Rail is a safety-critical, certified business. Plants work to welding-management systems such as EN 15085 and ISO 3834, and every length needs a record from billet to bundle. The line must support that paperwork, not fight it:

1. Heat and press log — alloy, billet, die and tonnage per length.

2. Quench record — temperature profile at exit and on the bed.

3. NDT check — ultrasonic or eddy-current for wall and seam.

4. Straightness and flatness scan before cut.

5. Certificate — per bundle, tied to the order and car number.

Our control cabinets log alloy, die, quench and saw data per bundle so the certificate writes itself. For the auxiliary units that make this possible, see the auxiliary equipment overview and the curtain-wall manufacturing article for how the same handling logic scales across segments.

Equipment implications for a rail line

Frequently asked questions

What profiles make up a rail or train car body?

The shell is built from large hollow sections: side walls, roof, floor, and the bodyside and bodyshell stringers, plus door and window surrounds. A single metro car uses roughly 8–12 t of extruded aluminium across 30–60 kg/m sections.

Why does rail extrusion need 8,000–12,000 t presses?

Car-body sections have a huge perimeter and wall area, so the extrusion force scales with perimeter, not mass. Sections up to 700–900 mm wide need 8,000–12,000 t to fill the die at a usable speed, and the container must take a large billet.

Why does handling become the bottleneck with 30–60 kg/m sections?

At 30–60 kg/m and 20–30 m long, a single length weighs 600–1,800 kg. The haul-off must pull it without sag, the cooling bed must support it full-length, and the saw must cut it square — so the auxiliary equipment, not the press, sets hourly output.

What flatness do friction-stir-weld car bodies need?

FSW joints need the mating faces flat and consistent, typically ≤0.2 mm flatness over the weld seam and ≤1 mm/m straightness, so the tool keeps full penetration. We set the straightener and saw to hold those numbers on every body-side length.

What traceability does rail extrusion quality require?

Rail work is covered by EN 15085 and ISO 3834 style welding-management systems, so every length needs a heat and press log, UT or eddy-current check, and a certificate. Our line logs alloy, die, quench and saw data per bundle for that record.