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A wheelset is a pair ofrailroad vehiclewheels mounted rigidly on anaxle. Wheelsets are often mounted in abogie ("truck" inNorth America) – a pivoted frame assembly holding at least two wheelsets – at each end of the vehicle. Most modernfreight cars andpassenger cars have bogies each with two wheelsets, but three wheelsets (or more) are used in bogies of freight cars that carry heavy loads, and three-wheelset bogies are under some passenger cars. Four-wheeledgoods wagons that were once near-universal inEurope andGreat Britain and their colonies have only two wheelsets; in recent decades such vehicles have become less common as trainloads have become heavier.
Mosttrain wheels have aconical taper of about 1 in 20 to enable the wheelset to followcurves with less chance of the wheelflanges coming in contact with the rail sides, and to reducecurve resistance. The rails generally slant inwards at 1 in 40, a lesser angle than the wheelcone. Without the conical shape, a wheel would tend to continue in a straight path due to theinertia of the rail vehicle, causing the wheelset to move towards the outer rail on the curve. The cone increases the effective diameter of the wheel as it moves towards the outer rail, and since the wheels are mounted rigidly on the axle, the outer wheels travel slightly farther, causing the wheelsets to more efficiently follow the curve. Abnormal wear at thewheel–rail interface is thus avoided,[1] along with the loud, piercing, very high-pitched squeal which usually results from it – especially evident on curves in tunnels, stations and elevated track, due to flat surfaces slipping and flanges grinding along the rail. However, if the degree of conicality is inappropriate for the suspension and track, an unpleasantoscillation can occur at high speeds. Recent research is also showing that marginal changes to wheel and rail profiles can improve performance further.[2]
Not all railroads have employed conical-tread wheels. TheBay Area Rapid Transit (BART) system inSan Francisco, built with cylindrical wheels and flat-topped rails, started to re-profile the wheels in 2016 with conical treads after years of complaints about thesqueal by its passengers.[3] Australia'sQueensland Railways used cylindrical wheels and vertical rails until the mid-1980s, when considerably higher train loads made the practice untenable.[4][5][6]
Somerubber-tyred metros feature special wheelsets withrubbertyres outside of deep-flanged steel wheels, which guide the bogie through standardrailroad switches and keep the train fromderailing if a tyre deflates. Thesystem was originally conceived byMichelin for theParis Métro; thefirst line opened in 1956.
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