Design studies for the second Turkmenistan – Afghanistan railway line have been completed, and work will start “soon”, the Afghan President’s office reports following a meeting between Presidents Karzai and Berdimuhamedov in Türkmenbaşy on 25 January 2012.

… President Karzai met Wednesday morning President Gurbanguly Berdimuhamedov to discuss a number of important economic projects including the TAPI, the Afghan-Turkmenistan railway and electricity export to Afghanistan.
[…]
Accompanying the President on the trip was Afghan Mining Minister, who said a design and full study of a separate project involving the establishment of an 84 km of railway extending across the Atamyrat-Ymamnazar in Turkmenistan to Akina-Andhoi in Afghanistan had been finalized with actual work expected to begin soon.

Source: Afghan and Turkmen Presidents Vow to Implement Critical Projects, Office of the President, Islamic Republic of Afghanistan, 26 January 2012

From Turkmenistan comes this report:

The Presidents also noted the great potential of cooperation in transportation and communications sector. They discussed the project on construction of the railway Atamyrat-Ymamnazar-Akin-Andhoy. A framework agreement on construction of this railway was signed during the official visit by the President of Turkmenistan to Afghanistan in May 2011. According to the interlocutors, construction and commissioning of the new railway will not only serve the development of transport infrastructure in Afghanistan but can become a significant element in the regional and international transit transportation in the long term.
Source: Turkmen-Afghan high level talks held in Turkmenbashi, Turkmenistan.ru, 26 January 2012

Here is a map I put together previously:

View Turkmenistan – Andkhoy railway plan in a larger map