機車軸式

在鐵路技術上，機車軸式，或稱機車輪式，是指用英文字母和阿拉伯數字的組合來表達鐵路機車走行部車軸排列結構和特點的系統化分類方式。世界上各地使用的機車軸式表達系統主要有三種。

• 華氏輪式：這是最早出現的機車軸式分類系統之一，廣泛應用於蒸汽機車的識別，主要為美國、加拿大、英國、愛爾蘭等地採用。
• AAR軸式：是一種由美國鐵路協會所制定的軸式分類法，主要應用於美國和加拿大的柴油機車和電力機車。
• UIC分類法：是一種由國際鐵路聯盟制定的軸式分類法，廣泛應用於歐洲和亞洲國家。

以華氏式別分類的蒸氣機車

1900年代的華氏式別手冊。

不同機車軸式之對比

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