mail car(n = noun.artifact) - a railway car in which mail is transported and sorted;
is a kind of car, railcar, railroad car, railway car
is a member of mail train

   A railway car of special design used for the transportation and sorting of mail en route to its destination, and having employees of the post office inside to perform those functions. [WordNet 1.5]

N  vehicle, conveyance, carriage, caravan, van, common carrier, wagon, waggon, wain, dray, cart, lorry, truck, tram, cariole, carriole, limber, tumbrel, pontoon, barrow, wheel barrow, hand barrow, perambulator, Bath chair, wheel chair, sedan chair, chaise, palankeen, palanquin, litter, brancard, crate, hurdle, stretcher, ambulance, black Maria, conestoga wagon, conestoga wain, jinrikisha, ricksha, brett, dearborn, dump cart, hack, hackery, jigger, kittereen, mailstate, manomotor, rig, rockaway, prairie schooner, shay, sloven, team, tonga, wheel, hobbyhorse, go-cart, cycle, bicycle, bike, two-wheeler, tricycle, velocipede, quadricycle, equipage, turn-out, coach, chariot, phaeton, break, mail phaeton, wagonette, drag, curricle, tilbury, whisky, landau, barouche, victoria, brougham, clarence, calash, caleche, britzka, araba, kibitka, berlin, sulky, desobligeant, sociable, vis-a-vis, dormeuse, jaunting car, outside car, dandi, doolie, dooly, munchil, palki, roller skates, skate, runabout, ski, tonjon, vettura, post chaise, diligence, stage, stage coach, mail coach, hackney coach, glass coach, stage wagon, car, omnibus, fly, cabriolet, cab, hansom, shofle, four-wheeler, growler, droshki, drosky, dogcart, trap, whitechapel, buggy, four-in-hand, unicorn, random, tandem, shandredhan, char-a-bancs, motor car, automobile, limousine, car, auto, jalopy, clunker, lemon, flivver, coupe, sedan, two-door sedan, four-door sedan, luxury sedan, wheels, sports car, roadster, gran turismo, jeep, four-wheel drive vehicle, electric car, steamer, golf cart, electric wagon, taxicab, cab, taxicoach, checker cab, yellow cab, station wagon, family car, motorcycle, motor bike, side car, van, minivan, bus, minibus, microbus, truck, wagon, pick-up wagon, pick-up, tractor- trailer, road train, articulated vehicle, racing car, racer, hot rod, stock car, souped-up car, bob, bobsled, bobsleigh, cutter, double ripper, double runner, jumper, sled, sledge, sleigh, toboggan, train, accommodation train, passenger train, express trail, special train, corridor train, parliamentary train, luggage train, freight train, goods train, 1st class train, 2nd class train, 3rd class train, 1st class carriage, 2nd class carriage, 3rd class carriage, 1st class compartment, 2nd class compartment, 3rd class compartment, rolling stock, horse box, cattle truck, baggage car, express car, freight car, parlor car, dining car, Pullman car, sleeping car, sleeper, dome car, surface car, tram car, trolley car, box car, box wagon, horse car, bullet train, shinkansen, cannonball, the Wabash cannonball, lightning express, luggage van, mail, mail car, mail van, shovel, spool, spatula, ladle, hod, hoe, spade, spaddle, loy, spud, pitchfork, post hole digger, tractor, steamshovel, backhoe, fork lift, earth mover, dump truck, bulldozer, grader, caterpillar, trench digger, steamroller, pile driver, crane, wrecking crane.

1st-class mail, air mail, armored car, armoured car, baggage car, buffet car, bulk mail, bumper car, cabin car, cable car, car, car battery, car bomb, car boot sale, car care, car carrier, car company, car dealer, car door, car factory, car horn, car insurance, car loan, car maker, car manufacturer, car mile, car mileage, car mirror, car park, car part, car pool, car port, car race, car racing, car rental, car seat, car sickness, car tire, car traffic, car train, car transporter, car wheel, car window, cattle car, chain mail, chair car, club car, coal car, coat of mail, compact car, dead mail, dining car, direct mail, drawing-room car, electric car, electronic mail, elevator car, estate car, express mail, fan mail, first-class mail, freight car, gondola car, grip car, hate mail, hire car, jaunting car, jaunty car, junk mail, lounge car, mail, mail bag, mail boat, mail call, mail carrier, mail clerk, mail coach, mail fraud, mail order, mail order house, mail out, mail pouch, mail service, mail slot, mail train, motor car, nonsmoking car, observation car, pace car, palace car, panda car, parlor car, parlour car, passenger car, patrol car, police car, prowl car, pullman car, race car, racing car, railroad car, railway car, refrigerator car, registered mail, ring mail, scout car, secondhand car, sleeping car, smoking car, snail mail, sport car, sports car, squad car, stock car, subcompact car, surface mail, tank car, third-class mail, touring car, tow car, trolley car, voice mail