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。

不同机车轴式之對比 

{| class="wikitable sortable"
|-class="background6"
!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
|}

機車軸式
分类系统