Many of us in India have vaguely heard of the line from Quetta to Zahidan in Iran, which was completed about a century ago. The location can be seen here in the north-west corner:
This map is from 1930 and only shows the line up to Nushki. This is a better map from the 1960s:
A good summary of its history up to 2007 by Owais Mughal can be seen here:
Also see the timetables from the 1930 NWR TT:
Note the distances: 68 miles (109 km) from Yakmach to Nok Kundi, a further 86 miles (138 km) to Mirjawa across the border and a further 52/84 to the terminus then called Duzdap.
And from the 1944 Bradshaw:
At that point the line was closed beyond Nok Kundi, though it was revived as part of the war effort.
Now we have something more up to date, in the form of 2 videos making a travelogue by a Pakistani blogger. These are fully in Urdu, though anyone who can understand Hindi should not have a problem.
The narrator’s style may be a little irritating, but you do see a good view of life on the Pakistan-Iran border including the last PR station at Koh-e-Taftan and the nearby road crossing along other bits like the last mosque and the last ATM in Pakistan. (This station seems to have come up after 1947).
Note the single train between Quetta and Zahidan which is a mixed train with very limited passenger space. It is interesting to see the large volume of Iranian consumer goods being imported here.
The latter video is not about the railways but concentrates on the manual transfer of goods across the border.
Footnote: we hear a lot of Gwadar and its port nowadays. Did you know that it was NOT part of British India but was an exclave of Oman until Pakistan took over in 1957, long after Independence.
Update in 2021:
Biking along this route from Taftan to Quetta in 2020.