- Main generator: In a diesel-electric locomotive, the main generator is coupled directly to the prime mover and its sole purpose is to feed electrical energy to the traction motors (an auxiliary generator powers lights and electrical appliances in the cab).
- Mainline (US) or Main line (UK): The principal artery of a railway system; cannot be occupied without authorization in the form of a movement permit (e.g. track warrant) or signal indication.
- Main reservoir: The compressed-air tank of a locomotive containing source air for the brakes and other pneumatic appliances.
- Main rod (US): The drive rod connecting the crosshead to a driving-wheel or axle in a steam locomotive. Connecting rod (UK).
- Maintenance of way (US): The maintenance of railroad rights of way, including track. Often abbreviated to MOW (as in MOW car).
- Mallet (pronounced "mallay"): type of articulated locomotive designed by Anatole Mallet. See "Compound Engine" above.
- Manifest: An express freight train carrying a variety of general merchandise.
- Matchbox tank (UK slang): a type of pannier tank where the tanks are square and do not rest of the locomotive frame.
- Mating worms (US): Penn Central logo (jargon/slang).
- Mechanical semaphore signal: A signal in which the aspect is conveyed by moving an arm.
- Merry-go-round (MGR) train (UK): coal train running between a coal mine and a power station, loading and unloading without stopping or shunting.
- Mikado: A steam locomotive with a 2-8-2 wheel arrangement.
- Milk Train: A aggregator for transporting milk from farms to dairies, such as British Railways Milk Trains
- MLW: Montreal Locomotive Works, bought by Bombardier and closed.
- Mogul: A steam locomotive with a 2-6-0 wheel arrangement.
- Motion (UK): Collective term for the connecting rod, coupling rods and valve gear; forms part of the running gear. Originally from Watt's invention of the parallel motion.
- Motor train (UK): See Auto train (UK) above.
- Mountain: A steam locomotive with a 4-8-2 wheel arrangement.
- Multiple aspect signalling: A system of colour-light signalling in which signals may show 3 or 4 aspects.
- Multiple unit (UK) MU (US): a self-propelled rail vehicle that can be joined with compatible others and controlled from a single driving station. The sub-classes of this type of vehicle; Diesel Multiple Unit (DMU), Diesel-Electric Multiple Unit (DEMU) and Electric Multiple Unit (EMU) are more common terms. These may also be termed railcars.
- Multiple-unit train control (US), Multiple working (UK): generally seen as the abbreviation MU, this normally refers to the ability of diesel and electric locomotives or multiple units to be joined together and controlled from one driving station. Such a set of joined locomotives is called (US) a consist or (colloquially) "lash-up" and is said to be "MUed together".
- Multiple working (UK): see Multiple unit (above).
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