Complex sentence: Phase two would extend the line from Spin Boldak to Kandahar, while phase three would extend from Kandahar to Herat and on to Khushka, Turkmenistan, thus linking the with the Central Asian.
Split into simpler sentences: Phase two would extend the line from Spin Boldak to Kandahar. Phase three would run from Kandahar to Herat and Khushka, Turkmenistan, linking the with the Central Asian.