Single-axle truck - Fig truck

...However, because the length of the body of a fixed-axle four-wheeled vehicle is limited in terms of curve-passing and running performance, when it becomes necessary to make the vehicle larger, a vehicle (bogie vehicle) was created that uses two sets of bogies that allow rotational freedom relative to the body. In addition, in order to improve running stability even with a short two-axle vehicle, a structure that allows a certain degree of relative movement (forward/backward and left/right) between the wheel set and the body has been adopted (this is sometimes called a single-axle bogie).
[Structure of the cart]
The smallest unit of the suspension is an axle with two pairs of wheels attached to it, called a wheel set. A bogie truck generally consists of two or more pairs of wheels, an axle box housing the axle bearings, a bogie frame that forms the framework of the truck, an axle spring that acts as a buffer between the axle box and the bogie frame, a cushion spring that acts as a buffer between the bogie frame and the car body, a center spring device that provides rotational freedom between the bogie frame and the car body, a rocker device that allows left-right movement between the bogie frame and the car body, and a brake device (see figure).

*Some of the terminology that refers to "single-axle bogies" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…しかし，2軸固定式の四輪車では曲線通過性能と走行性能の面から車体長に限界があるため，車両の大型化が必要となると，車体に対して回転の自由度をもたせた台車(ボギーbogie台車)を2組用いた車両(ボギー車)が生まれた。また車体長の短い2軸車でも走行安定性を向上させるため，輪軸と車体の間にはある程度の相対運動(前後，左右方向)が可能な構造が採られるようになってきている(これを1軸台車と呼ぶことがある)。
［台車の構造］
　足まわりの最小単位として1本の車軸に2対の車輪を取り付けたものを輪軸と呼ぶが，ボギー台車は一般に，2対以上の輪軸，車軸用軸受を収納した軸箱，台車の骨組をなす台車枠，軸箱と台車枠との間で緩衝作用をする軸ばね，台車枠と車体との間で緩衝作用をする枕ばね，台車枠と車体の間に回転の自由度を与える心皿装置，台車枠と車体の間の左右方向の動きを許容する揺れ枕装置，ブレーキ装置などで構成される(図)。…

※「１軸台車」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報