A train has no steering wheel. It turns because its wheels are slightly conical, and it takes curves because its wheels sit on carriages that pivot under the body: the bogies.
What it is
A bogie (called a truck in North America) is a carriage with two axles, sometimes three, placed under each end of a coach or locomotive. The body rests on it through a pivot: the bogie can turn under it. That's what lets a twenty-metre vehicle follow a curve, while its wheels stay parallel to the rails.
A freight wagon or passenger coach has two bogies, so four axles and eight wheels. A bogie weighs several tonnes.
The wheelset: why a train turns without steering
The two wheels of an axle are fixed to the same shaft. They always turn together, at the same speed. In a curve, the outer wheel has further to travel: how does it manage?
The tread is conical. When the wheelset shifts towards the outside of the curve, the outer wheel rolls on a larger diameter and the inner wheel on a smaller one. The outer wheel then covers more ground per turn, and the wheelset centres itself. That's what guides the train, far more than the flanges.
The flange, the rim on the inside of each wheel, is only a safety: it should only touch the rail in tight curves and at switches. When it rubs, you hear the high screech of trains in curves.
The suspensions
- Primary suspension, between the axle boxes and the bogie frame: metal springs or rubber blocks. It absorbs track defects.
- Secondary suspension, between the bogie and the body: often air springs on passenger trains. That's what makes the ride comfortable.
What goes on a bogie
- Brakes: discs on the axle, or blocks rubbing on the tread.
- Traction motors, on the powered bogies of locomotives and multiple units. The others are called trailer bogies.
- Sensors for speed and axle box temperature, and the beacons of the speed control system.
Variants
- The Jacobs bogie is shared between two cars, at their joint. It's the principle of articulated trainsets like the TGV: fewer bogies, a more stable and lighter train.
- Variable-gauge bogies change the distance between their wheels while rolling slowly through a special installation, to pass from one network to another, between Spain and France for example.
- Trams and metros have smaller bogies for much tighter curves.
Tips from people who maintain them
- Hunting: above a certain speed, a conical wheelset swings left and right more and more. The whole design of a high-speed bogie aims to damp it. A worn bogie that "hunts" is felt in the body as a regular sway.
- Wheels are reprofiled without being removed: the train rolls over an underfloor lathe that shaves the tread back to its conical profile.
- A "flat" on a wheel, made when the wheel skidded locked under braking, makes a regular hammering noise: the famous "clack-clack-clack" of a train arriving with a damaged wheel.
- Axle boxes heat up when their bearing is dying. Trackside detectors measure their temperature as trains pass, and stop the train before the axle breaks.
The track all this rolls on is described in Railway track.