▁of ▁standard ▁gauge ▁track , ▁such ▁as ▁height ▁of ▁over pass es ▁so ▁that ▁trains ▁can ▁be ▁ex changed . ▁The ▁choice ▁of ▁train ▁cou pl ers ▁may ▁be ▁a ▁factor ▁as ▁well . ▁ ▁Rol ling ▁stock ▁ ▁Where ▁vehicles ▁move ▁to ▁a ▁different ▁gauge , ▁they ▁must ▁either ▁be ▁prepared ▁for ▁bog ie ▁exchange ▁or ▁be ▁prepared ▁for ▁wheel set ▁exchange . ▁For ▁example , ▁passenger ▁trains ▁moving ▁between ▁the ▁ ▁in ▁France , ▁and ▁the ▁ ▁gauge ▁in ▁Spain , ▁now ▁pass ▁through ▁an ▁installation ▁which ▁adjust s ▁their ▁variable ▁gauge ▁ax les . ▁This ▁process ▁is ▁known ▁as ▁" g au ge ▁change ". ▁Good s ▁w ag ons ▁are ▁still ▁subject ▁to ▁either ▁bog ie ▁exchange ▁or ▁wheel set ▁exchange . ▁ ▁Ste am ▁locomot ives ▁F ew ▁steam ▁locomot ives ▁are ▁constructed ▁to ▁allow ▁for ▁gauge ▁change . ▁Those ▁that ▁were ▁include ▁some ▁in ▁East ▁African ▁Rail ways , ▁G arr att s , ▁and ▁the ▁Victor ian ▁Rail ways ▁R ▁class ▁R 7 6 6 . ▁Const ruction ▁of ▁steam ▁locomot ives ▁in ▁this ▁way ▁is ▁difficult ▁due ▁to ▁the ▁width ▁of ▁the ▁bo ilers ▁and ▁the ▁fire box es . ▁The ▁gauge ▁change ▁is ▁only ▁to ▁a ▁wider ▁gauge ▁because ▁of ▁the ▁width s ▁of ▁the ▁bo ilers ▁and ▁fire box es ▁on ▁the ▁s ▁ ▁locomot ives ▁as ▁opposed ▁to ▁those ▁on ▁. ▁▁ ▁In ▁about ▁ 1 8 6 0 , ▁the ▁Br istol ▁and ▁Ex eter ▁Railway ▁converted ▁five ▁ ▁locomot ives