The contract is part of more than $300 million that Abu Dhabi-based GAAC aims to invest in Afghanistan's aviation sector over 10 years.
A pilot flying at the same time the Ukrainian airliner was hit reported seeing 'the light of a missile' and then 'an explosion'.
An upcoming Tashkent summit of the two countries' presidents could determine the fate of the railway project, say observers.
Kabul's more than 6 million residents presently deal with epic traffic jams and hope to see some relief in a year.
The Turkmenistan-Afghanistan-Tajikistan (TAT) railway has already brought a number of Turkmen commodities to Afghan markets.