ically ▁employed ▁at ▁large ▁railway ▁stations ▁and ▁in ▁goods ▁yards , ▁they ▁tap ▁whe els ▁with ▁a ▁long - hand led ▁ham mer ▁and ▁listen ▁to ▁the ▁sound ▁made ▁to ▁determine ▁the ▁integrity ▁of ▁the ▁wheel ; ▁crack ed ▁whe els , ▁like ▁crack ed ▁b ells , ▁do ▁not ▁sound ▁the ▁same ▁as ▁their ▁int act ▁counter parts ▁( they ▁do ▁not ▁" ring ▁true "). ▁ ▁Whe elt app ers ▁also ▁check ▁that ▁the ▁ax le ▁boxes ▁are ▁not ▁over ly ▁hot ▁by ▁using ▁the ▁back ▁of ▁their ▁hand . ▁ ▁Although ▁whe elt app ers ▁still ▁operate ▁in ▁some ▁eastern ▁European ▁countries , ▁in ▁countries ▁with ▁modern ▁planned ▁maintenance ▁procedures ▁and ▁line - side ▁defect ▁detect ors , ▁such ▁as ▁hot ▁box ▁detect ors , ▁whe elt app ers ▁are ▁redundant . ▁The ▁job ▁is ▁mostly ▁associated ▁with ▁the ▁steam ▁age . ▁Whe elt app ers ▁were ▁vital ▁to ▁the ▁smooth ▁running ▁of ▁the ▁rail ways ▁as ▁a ▁crack ed ▁wheel ▁or ▁over he ated ▁ax le ▁bearing ▁would ▁lead ▁to ▁del ays ▁and ▁the ▁loss ▁of ▁re venue . ▁These ▁were ▁particularly ▁common ▁in ▁the ▁ 1 9 th ▁century , ▁when ▁ax le ▁bear ings ▁were ▁lub ric ated ▁by ▁gre ase . ▁At ▁this ▁time , ▁met all ur gy ▁was ▁a ▁more ▁ha ph az ard ▁science ▁and ▁thus ▁it ▁was ▁impossible ▁to ▁test ▁steel ▁whe els ▁for ▁cra cks : ▁the ▁role ▁of ▁the ▁whe elt apper ▁was ▁of ▁cru cial ▁importance . ▁ ▁A