在铁路技术上，机车轴式，或称机车轮式，是指用英文本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。

In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country. Within a given country, different notations may also be employed for different kinds of locomotives, such as steam, electric, and diesel powered.