Bogies & suspension systems
A bogie is the wheeled underframe assembly on which a rail vehicle carbody rests, providing guidance along the track and isolating the vehicle structure from wheel–rail interaction forces.
A standard bogie consists of a steel frame carrying two wheelsets, primary suspension between the axle boxes and the bogie frame, secondary suspension between the bogie frame and the carbody, and a braking arrangement acting on the wheels or axle-mounted discs.
The bogie transfers traction and braking forces longitudinally through centre pivot or traction links, and lateral forces through secondary suspension side stops. Steered bogies, used on some high-speed and articulated designs, actively or passively adjust wheelset yaw to reduce flange forces in curves.
Two-axle bogies are standard on most European passenger rolling stock. Three-axle bogies appear on high-axle-load freight locomotives where load distribution requires it.
Primary and secondary suspension
Primary suspension acts between the axle box and the bogie frame. Coil springs, rubber-metal conical elements, or a combination of both are used at this stage; the function is to absorb high-frequency wheel–rail irregularities and limit axle box movement relative to the frame.
Secondary suspension acts between the bogie frame and the carbody, providing ride comfort and lateral stability. Air springs are standard on modern passenger stock: they maintain constant carbody height under varying loads by adjusting internal pressure, and provide a lower vertical natural frequency than steel springs, improving ride comfort.
Running dynamics and standards
Bogie performance is assessed against EN 14363, which defines limits for wheel–rail forces, body accelerations, and derailment safety criteria. The Nadal criterion governs the ratio of lateral (Y) to vertical (Q) wheel forces; exceedance indicates derailment risk. Dynamic tests cover normal track irregularities across the operational speed range and are conducted either on track or on roller rigs. EN 13749 specifies structural requirements and fatigue assessment methods for bogie frames.
Bogie mass typically falls in a range of several tonnes per unit, varying with traction equipment and braking configuration. Minimising unsprung mass — the components rotating with the wheelset — reduces track forces and wear at high speed, and is a primary design constraint on motor and brake disc mounting arrangements.
Active suspension
Electro-hydraulic and electro-pneumatic active lateral suspension systems are in service on selected high-speed platforms, applying corrective forces to the carbody in curves to reduce passenger lateral acceleration. Active systems allow higher speed through curves without increasing track forces or requiring cant excess. Lateral acceleration limits for standing passengers in EN 13848 and TSI LOC&PAS define the operational design target.