MAGNETIC FLOAT GAUGE
Usually a closed system gauging device that uses magnets to couple a liquid float to a visually observed gauge rod. See "B-612" and "Gauging Device."

MAJOR CLASSES
See Dot Class.

MALE CENTER PLATE
See "Body Center Plate."

MANWAY
A general term designating the opening and surrounding parts located at the top of each tank permitting worker access to the tank interior.

MANWAY COVER
The hinged and bolted, or totally bolted cover that seals the manway opening.

MANWAY NOZZLE
The cylindrical shape that is welded to the top of each tank defining the manway opening and upon which the manway cover is mounted.

MARKED
Information applied to a car by either stencil or decal.

MARKED CAPACITY
The volumetric (gallons) shell full capacity stenciled on each end of a tank car.

MARKET LEASE RATE
The lowest monthly lease rate generally available at any particular time to lessees of railcars from the various leasing companies. This rate will also vary depending on the type of car in question and other factors.

MAXIMUM GROSS WEIGHT ON RAILS
See "Gross Weight."

MAXIMUM WIDTH
The maximum railcar width as determined by the applicable clearance diagram.

MECHANICAL DESIGNATION
An alphabetic code assigned by the AAR to each freight car to designate its general design characteristics and its intended purpose.

MICROWAVE GAUGING DEVICE
A closed-system gauging device using microwave technology rather than mechanical floats, direct visual, ultrasonic, or other means to detect tank liquid level.

MILEAGE
Rail miles travelled by a car, based upon distance calculated between two points by various railroads.

MILEAGE ALLOWANCE
The allowance paid by railroad to non-railroad car owners.

MILEAGE CREDITS
A rental credit for loaded movements by rail, based on loaded miles travelled multiplied by an applicable mileage rate.

MILEAGE EQUALIZATION
An empty mileage discipline program for tank cars in the US and tank and hopper cars in Canada that penalizes car owners for accumulating empty miles in excess of a set percentage of loaded miles.

MILEAGE RATE
Rates applicable according to distance.

MINIMUM THICKNESS
Thickness of material required by regulations and/or design of the car to safely transmit in-line train forces.

MOBILE REPAIR UNIT
A crew that travels to a particular location to make repairs without the car being shopped.

MODE
A term used to refer to the basic means of transport; rail, truck, air, and water.

MSDS SHEETS
Material Safety Data Sheet required to be provided by chemical manufacturers & shippers to potential handlers & users of the products.

MUD GUARDS
Relatively light weight fender-type structures mounted above the wheels of some tank cars to prevent foreign matter from being thrown against and thereby dirtying the car body.

MULTI UNIT TANK CAR
Refers to tank cars built to DOT 106A or 110A specs. Consisting of a series of small tanks designed to be removed from the car for filling and emptying.

MULTIPLE HOUSING
A term sometimes used to refer to the top unload fittings on a tank car.

MULTIPLE WEAR WHEEL
A railcar wheel originally made with extra rim thickness thereby allowing a worn wheel to be machined and restored to its original shape at least twice during its life.