▁Mountains ▁of ▁Imperial ▁County . ▁The ▁quar ry ▁is ▁estimated ▁to ▁contain ▁a ▁depos it ▁of ▁ 2 5 ▁million ▁tons ▁of ▁gy psum . ▁ ▁US G ▁oper ates ▁an ▁active ▁narrow ▁gauge ▁railway , ▁the ▁last ▁industrial ▁narrow ▁gauge ▁railway ▁in ▁the ▁United ▁States . ▁ ▁The ▁ ▁gauge ▁line ▁runs ▁north ▁for ▁ ▁from ▁the ▁plant ▁at ▁Pl aster ▁City ▁( former ly ▁known ▁as ▁Maria ) ▁to ▁the ▁gy psum ▁quar ry . ▁ ▁The ▁line ▁ha uls ▁gy psum ▁rock ▁from ▁the ▁quar ry ▁to ▁the ▁plant . ▁ ▁The ▁line ▁was ▁originally ▁built ▁by ▁the ▁Imperial ▁Gy psum ▁Company ▁Rail road ▁and ▁was ▁owned ▁by ▁the ▁ ▁Imperial ▁Valley ▁Gy psum ▁and ▁O il ▁Corporation . ▁ ▁The ▁rail road ▁built ▁from ▁the ▁San ▁Diego ▁& ▁Arizona ▁Railway ▁at ▁Pl aster ▁City ▁to ▁the ▁quar ry . ▁ ▁Sur ve ying ▁comm enced ▁in ▁April ▁ 1 9 2 1 , ▁grad ing ▁on ▁October ▁ 3 , ▁ 1 9 2 1 ▁and ▁construction ▁was ▁completed ▁on ▁September ▁ 1 5 , ▁ 1 9 2 2 . ▁ ▁Com mer cial ▁operation ▁comm enced ▁on ▁October ▁ 1 4 , ▁ 1 9 2 2 . ▁ ▁The ▁total ▁length ▁of ▁the ▁line ▁was ▁. ▁Two ▁years ▁after ▁completion ▁of ▁the ▁line ▁( 1 9 2 4 ), ▁the ▁track ▁was ▁sold ▁to ▁the ▁Pacific ▁Port land ▁C ement ▁Company . ▁ ▁US G ▁purchased ▁the ▁line ▁from ▁the ▁Pacific ▁Port land ▁C ement ▁Company