A 26 February 2009 report in Pakistan’s Daily Times quotes Uzbekistan’s president Islam Karimov as having said the previous day that a railway is being built to Mazar-i-Sharif:

“Uzbekistan has agreed to allow non-military, I underline, non-military cargo to be transited through Uzbek territory to Afghanistan, in accordance with existing Uzbek legislation,” Karimov told reporters. “Uzbekistan is participating in the development of the communication and transport infrastructure of Afghanistan. We’ve started a construction project on a railway from the (Uzbek) city of Termez to (the northern Afghan city of) Mazar-e Sharif,” he said.
Source: Daily Times 2009-02-26

Termez is the nearest Uzbek city to the Friendship Bridge, and Uzbek Railways already operates to the freight terminal at Hayratan just inside Afghanistan. An extension of the line on to Mazar-i-Sharif has long been proposed, but is it really happening? I can’t spot anything (in English) on the presidential website, and the quote doesn’t seem to appear in other versions of essentially the same story.