From Sibi the line runs south-west, skirting the hills to Rindli, and originally followed the course of the Bolān stream to its head on the plateau.
The destructive action of floods, however, led to the abandonment of this alignment, and the railway now follows the Mashkaf valley (which debouches into the plains close to Sibi), and is carried from near the head of the Mashkaf to a junction with the Bolān at Machh.
An alternative route from Sibi to Quetta was found in the Harnai valley to the N.E. of Sibi, the line starting in exactly the opposite direction to that of the Bolān and entering the hills at Nari.
The Harnai route, although longer, is the one adopted for all ordinary traffic, the Bolān loop being reserved for emergencies.
At the Khundilani gorge of the Bolān route conglomerate cliffs, which rise to a height of 800 ft.,
enclose the valley.
 At Siri Bolān the passage between the limestone rocks hardly admits of three persons riding abreast.
The temperature of the pass in summer is very high, whereas in winter, near its head, the cold is extreme, and the ice-cold wind rushing down the narrow outlet becomes destructive to life.
Since 1877, when the Quetta agency was founded, the pass was secured by the British Indian Army from militias of Baluch tribesmen (chiefly Marris).
From Sibi the line runs south-west, skirting the hills to Rindli, and originally followed the course of the Bolān stream to its head on the plateau. The destructive action of floods, however, led to the abandonment of this alignment, and the railway now follows the Mashkaf valley (which debouches into the plains close to Sibi), and is carried from near the head of the Mashkaf to a junction with the Bolān at Machh. An alternative route from Sibi to Quetta was found in the Harnai valley to the N.E. of Sibi, the line starting in exactly the opposite direction to that of the Bolān and entering the hills at Nari. The Harnai route, although longer, is the one adopted for all ordinary traffic, the Bolān loop being reserved for emergencies. At the Khundilani gorge of the Bolān route conglomerate cliffs, which rise to a height of 800 ft., enclose the valley.  At Siri Bolān the passage between the limestone rocks hardly admits of three persons riding abreast. The temperature of the pass in summer is very high, whereas in winter, near its head, the cold is extreme, and the ice-cold wind rushing down the narrow outlet becomes destructive to life. Since 1877, when the Quetta agency was founded, the pass was secured by the British Indian Army from militias of Baluch tribesmen (chiefly Marris).
Bolan Pass