机车轴式

在铁路技术上，机车轴式，或称机车轮式，是指用英文字母和阿拉伯数字的组合来表达铁路机车走行部车轴排列结构和特点的系统化分类方式。世界上各地使用的机车轴式表达系统主要有三种。

• 华氏轮式：这是最早出现的机车轴式分类系统之一，广泛应用于蒸汽机车的识别，主要为美国、加拿大、英国、爱尔兰等地采用。
• AAR轴式：是一种由美国铁路协会所制定的轴式分类法，主要应用于美国和加拿大的柴油机车和电力机车。
• UIC分类法：是一种由國際鐵路聯盟制定的轴式分类法，广泛应用于欧洲和亚洲国家。

以華氏式別分類的蒸氣機車

1900年代的华氏式别手册。

不同机车轴式之對比

UIC軸式 VDEV/VMEV/UIC-System	華氏式別 （Whyte-Notation）	美式稱呼	簡單圖解 左方為機車前進方向
A1	0-2-2		Oo
A2	0-2-4		Ooo
1A	2-2-0	Planet	oO
1A1	2-2-2	Patentee	oOo
1A2	2-2-4		oOoo
2′A	4-2-0	Crampton, Norris, Jervis	ooO
2′A1	4-2-2	Single Driver	ooOo
2A2	4-2-4		ooOoo
3A	6-2-0	Crampton	oooO
N/A	0-3-0		OOO
B	0-4-0	Four-Wheel-Switcher	OO
B1	0-4-2		OOo
B2′	0-4-4	Forney	OOoo
B3′	0-4-6		OOooo
1B	2-4-0	Hanscom	oOO
1′B1′	2-4-2	Columbia	oOOo
1B2′	2-4-4		oOOoo
1B3′	2-4-6		oOOooo
2′B	4-4-0	American, Eight-Wheeler	ooOO
2′B1′	4-4-2	Atlantic	ooOOo
2′B2′	4-4-4	Jubilee	ooOOoo
2′B3′	4-4-6		ooOOooo
C	0-6-0	Six-Wheel-Switcher	OOO
C1	0-6-2		OOOo
C2′	0-6-4		OOOoo
1′C	2-6-0	Mogul	oOOO
1′C1′	2-6-2	Prairie	oOOOo
1′C2′	2-6-4	Adriatic	oOOOoo
1′C3′	2-6-6		oOOOooo
2′C	4-6-0	Ten-Wheeler	ooOOO
2′C1′	4-6-2	Pacific	ooOOOo
2′C2′	4-6-4	Hudson, Baltic	ooOOOoo
D	0-8-0	Eight-Wheel-Switcher	OOOO
D1	0-8-2		OOOOo
D2′	0-8-4		OOOOoo
D3′	0-8-6		OOOOooo
1′D	2-8-0	Consolidation	oOOOO
1′D1′	2-8-2	Mikado	oOOOOo
1′D1′	2-8-2T	MacArthur	oOOOOo
(1′D1′)(1′D1′)	2-8-2+2-8-2	Double Mikado, Garrett	oOOOOo+oOOOOo
1′D2′	2-8-4	Berkshire	oOOOOoo
1′D3′	2-8-6		oOOOOooo
2′D	4-8-0	Twelve-Wheeler, Mastodon	ooOOOO
2′D1′	4-8-2	Mountain, Mohawk (NYC)	ooOOOOo
2′D2′	4-8-4	Northern, General Service (SP), Golden State (SP), Niagara (NYC), Wyoming	ooOOOOoo
2D3	4-8-6		ooOOOOooo
3′D3′	6-8-6	nameless (Pennsylvania Railroad Steam Turbine)	oooOOOOooo
E	0-10-0	Ten-Wheel Switcher	OOOOO
E1′	0-10-2	Union	OOOOOo
1′E	2-10-0	Decapod	oOOOOO
2′E	4-10-0	Mastodon	ooOOOOO
1′E1′	2-10-2	Santa Fe	oOOOOOo
1′E2′	2-10-4	Texas	oOOOOOoo
2′E1′	4-10-2	Southern Pacific, Overland	ooOOOOOo
F	0-12-0	Pennsylvania, Twelve-Wheel-Switcher	OOOOOO
1′F	2-12-0	Centipede	oOOOOOO
1′F1′	2-12-2	Javanic	oOOOOOOo
2′F1′	4-12-2	Union Pacific	ooOOOOOOo
2′G2′	4-14-4		ooOOOOOOOoo
nameless (AA)	0-2-2-0		O O
B′B	0-4-4-0	nameless (Mallet)	OO OO
B′B1	0-4-4-2	nameless (Mallet)	OO OOo
2′BB2′	4-4-4-4	nameless (Pennsylvania Railroad Duplex)	ooOO OOoo
3′BB3′	6-4-4-6		oooOO OOooo
2′CB2′	4-6-4-4	nameless (Pennsylvania Railroad Duplex)	ooOOO OOoo
C′C	0-6-6-0	Erie (Mallet)	OOO OOO
(1′C)C	2-6-6-0	nameless (Mallet)	oOOO OOO
(1′C)C1′	2-6-6-2	Mallet Mogul (SP), Prairie Mallet (ATSF)	oOOO OOOo
(1′C)C2′	2-6-6-4	nameless (Simple articulated)	oOOO OOOoo
(2′C)C2′	4-6-6-4	Challenger (Simple articulated)	ooOOO OOOoo
(1′C)C3′	2-6-6-6	Allegheny (Mallet/Simple articulated)	oOOO OOOooo
D′D	0-8-8-0	Angus (Mallet)	OOOO OOOO
(1′D)D	2-8-8-0	Bullmoose (Mallet)	oOOOO OOOO
(1′D)D1′	2-8-8-2	Chesapeake, Mallet consolidation (Mallet)	oOOOO OOOOo
(1′D)D2′	2-8-8-4	Yellowstone (Mallet/Simple articulated)	oOOOO OOOOoo
(2′D)D1′	4-8-8-2	Cab Forward (Simple articulated)	ooOOOO OOOOo
(2′D)D2′	4-8-8-4	Big Boy (Simple articulated)	ooOOOO OOOOoo
(1′E)E1′	2-10-10-2	Virginian (Mallet)	oOOOOO+OOOOOo
(2′C1′)(1′C2′)	4-6-2+2-6-4	Double Pacific (Garratt)	ooOOOo+oOOOoo
(2′C2′)(2′C2′)	4-6-4+4-6-4	Double Hudson (Garratt)	ooOOOoo+ooOOOoo
(2′D)(D2′)	4-8-0+0-8-4	Double Mastodon (Garratt)	ooOOOO+OOOOoo
(2′D1′)(1′D2′)	4-8-2+2-8-4	Double Mountain (Garratt)	ooOOOOo+oOOOOoo
(2′D2′)(2′D2′)	4-8-4+4-8-4	Double Northern (Garratt)	ooOOOOoo+ooOOOOoo