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Madhouse 1) Engine foreman. 2) A scene of unusual activity or confusion.
Main Line (also Main Iron, Main Stem, Main Track) Primary rail line over which trains operate between terminals. It excludes sidings, and yard and industry tracks. During the Timetable / Train Order era, the main line was  restricted by rules to travel only by scheduled trains or those trains with train orders or on a schedule. Scheduling is now an exception and not a rule, and most lines are operated by Track Warrant or CTC or other systems.
Maintenance Of Way (MW or MOW) A location where work is being done by Roadway Crews.
Maintenance Of Way Equipment The machinery and rolling stock used to keep track and roadbed in good condition.
Main Pin An official.
Make A Joint Couple cars.
Mallet An articulated steam locomotive named for the designer, Anatole Mallet. Sometimes used to describe any articulated locomotive, though "true" Mallets were compound engines.
Malley Reference to the Mallet Articulated Cab Forward steam locomotives used by Southern Pacific railroad in the 30's, 40's and 50's.
Manifest Fast freight usually made up of mixed railcars.
Manual Block System A series of consecutive blocks, governed by block signals operated manually, using information by telegraph, telephone or other means of communication.
Marker A reflector flag, light, or other highly visible marking device, in the red-orange-amber color range, affixed to the rear of a train to indicate that the train is complete. Markers may be communicating or non-communicating devices. As an added note, rear helper engines may serve as markers.
Marklin Marklin is a German manufacturer of high quality precision model trains. Marklin is the worlds largest manufacturer of model trains, and produces "Z" scale, "N" scale "HO" scale and #1 Gauge large scale trains.
Mars Light An oscillating light on the front of a road locomotive.
Marshaling Yard Yard (classification yard) where trains are disconnected and reassembled based on their destination.
Master Maniac Master mechanic.
Master Mind An official.
Matching Dials Comparing time.
Maul Work an 'engine with full stroke and full throttle.
McKeen Cars Gasoline-mechanical passenger railcars, built between 1905 and 1917. They had distinctive steel bodies, with a "windsplitter" front end, round tail, port-hole windows and center entrance.
ME Maintenance of Equipment Department.
Meat Run Fast run of perishable freight, hotshot.
Meet When two trains traveling in opposite directions pass each other. Usually used to describe a single mainline operation where one train waits on a siding for the other to pass.
Meet Order Train order or other instructions specifying when and where two trains should meet and which one should remain on the main.
Merry-Go-Round Turntable.
Mexico Crossings Number of cars delivered/received on a daily basis between U.S. railways and Mexican railways at border points.
Micro-Trains Manufacturer of precision model railroading equipment in both "Z" scale and "N" scale.
Middle Man, Middle Swing Second brakeman on freight train.
Mike Mikado-type engine (2-8-2), so named because first of this type were built for Imperial Railways of Japan. (Because of the war with Japan, some railroads re-christened this type MacArthur)
Mile Post (MP) Any point along the right-of-way, designated by a measurement from a specific location given in tenths of a mile, i.e. MP 764.2. Also, a trackside sign displaying this measurement.
Mileage Hog Engineer or conductor, paid on mileage basis, who uses his seniority to the limit in getting good runs.
Milk Car Refrigerator car for milk.
Milk Train A slow train.
Milk Truck Large hand truck with high cast-iron wheels used to transfer milk cans around in a terminal.
Mill 1) Steam locomotive.  2) Typewriter.
Mixed Load Truckload of mail sacks and parcels for many destinations sent from storage car to the yard (an outside platform) for further separation before forwarding.
Mixed Train A train pulling both passengers and freight cars.
Module A section of a layout that is built following a standard pattern or dimensions. Each module can be connected interchangeably with any other module built to the same standards. NMRA has developed standards for "HO" and Ntrak is an organization that has developed standards for N scale modules.
Monkey 1) When a crew has been on duty sixteen hours and is caught out on the road, the monkey gets them and they are required by Federal labor laws to tie up until a new crew comes. (See dogcatchers) 2) Monkey House: Caboose. 3) Monkey Money: The pass of a passenger who is riding free. 4) Monkey Motion: Walschaert or Baker valve gear on locomotive. 5) Monkey Suit: A passenger trainman's uniform or any other smart-looking uniform. 6) Monkey's Tail: Slang for the handle of a switch stand, as in twisting the monkey's tail.
Monorail A railroad in which the train runs on a single rail.
Moonlight Mechanic Night roundhouse foreman.
Moped [SL] Class 342 B-B Asgen electrics. So called because of their light weight and incapability to drag heavy trains.
Mopping Off Refers to escaping steam.
Mother Hubbard A locomotive with the cab straddling the boiler like a saddle. (See Camelback)
Motion A moving mechanism (valve gear) on a steam locomotive.
Motor Electric locomotive.
Motor Bridge Bogie having driving wheels or motored axles.
Motor Car A motor-driven railway inspection or work car which rides on the rails and is operated by maintenance of way employees to minimize time spent traveling while on duty. NOTE Motor-Driven actually means driven by a small gasoline or diesel engine.
Motorman Driver of an electric train, railcar trolley, streetcar or multiple unit train.
Mountain Pay Overtime.
Mountain Railway Specialized form of railway for ascending mountains.
Movable Point Frog A frog with a moving alignment allowing open flangeway for train movement.
Moving Dirt Fireman shoveling coal into firebox.
Moving Spirit Train dispatcher, more often called DS.
MP 1) Mile Post. 2) Motive Power Department
MRIA Model Railroad Industry Association, An association of Model Railroad manufacturers.
MTH Mikes Train House "O" gauge model railroad manufacturer.
MTYS Empty cars.
MU See Multiple Unit.
Muckers Excavators in construction work.
Mud Hen A surveyor.
Mud Hop A yard clerk. A car checker who maintains a listing of freight cars on arriving and departing trains.
Mud Sucker A non-lifting injector.
Mule A movable, hand-carried derail that is placed on either rail.
Mule Skinner Driver of mule cart.
Multilevel Car A long flatcar designed with one or more deck levels in addition to the car's main deck; used to haul new automobiles and trucks.
Multiple Aspect Signaling (MAS) A system of color light signaling, that could be provided either by multi-lens, or searchlight signals in which each signal unit can display more than two aspects.
Multiple Main Tracks Two or more main tracks, the use of which is designated in the timetable.
Multiple Unit (MU) Cars or locomotives which contain their own power but which can be controlled from the foremost car or locomotive Used on commuter trains and diesel locomotives. Cables between units link controls in "party line fashion".
Music Master Paymaster.
Mutt And Jeff Pump Denver & Rio Grande locomotive with big air pump on right and small one on left.
Muzzle Loader Any hand-fired steam locomotive.
MW Maintenance of Way Department