Starting in 2008 the U.S. build an alternative logistic route into Afghanistan. Instead of trucks traversing Pakistan the Northern Distribution Network would depend on the railway system of Uzbekistan, Tajikistan and other former Soviet Union states.

That railway system was recently extended into Afghanistan with the termination point in Mazar-e-Sharif. It is overall a fragile and long route depending on old infrastructure in a dubious maintenance state. In Uzbekistan trains take priority only when various bribes get paid to the right people.

Still the plans are to in future route 75% of all land based logistics through the NDS to become less dependent on Pakistan. This again could eventually open options to put more political or military pressure on Pakistan.

It is obvious that the situation would make the NDS a target for several constituencies and now the inevitable happened:

Uzbekistan media have reported an explosion on a railway line on the Central Asian nation’s border with Afghanistan.

The rail route lies on a distribution network used for the supply of goods to United States troops serving in Afghanistan, but the cause of the blast was unclear.
…
Privately operated Uzmetronom news site, based in the Uzbek capital, Tashkent, said investigators are probing a possible terrorist link.

A bit more, though still murky information via Eurasianet:

There is very little information about this so far, but there hasn't been a terror attack in Uzbekistan for several years. And the fact that it's so near to Termez, the hub of the U.S.'s Northern Distribution Network that carries military cargo through Central Asia to Afghanistan, has to have people worrying in Tashkent and the Pentagon.
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A map on the Ferghana News site shows where Galaba and Amu Zang are, and it is in fact on a line that does coincide with the NDN. And a press release on the Russian Railways site (in Russian) says that service on the line is disrupted because of the "destruction of the supports of the railway bridge."

The incident happened on Wednesday and was only made public today. With a bridge down the repair may take some time.

It is easy to blame "terrorists" for the incident. But there are many groups who could have a possible interests here.

• The Taliban are a possible candidate. Attacking the logistics of their enemies is a preferred choice for them.
• The Pashtun trucker mafia from Karachi is providing most of the trucks used on the supply line through Pakistan. It is losing business to the NDS and breaking it would keep their revenue stream up.
• The Pakistani military is interested in keeping a decisive hand over the endgame in Afghanistan. Its pressure point it currently the U.S. logistic line through Pakistan. Losing that is not in its strategic interest.
• All of the above would possible use surrogates from the reinstated Islamic Movement of Uzbekistan, which, allegedly, has some groups in eastern Pakistan, to operate in Uzbekistan.
• Besides the IMU there are possibly other groups with an interest in overthrowing the brutal Usbek dictatorship of Islam Karimov and who could use such attacks decrease his utility for his U.S. supporters.
• Last but not least Karimov himself and the various Uzbek criminals involved in profiting from the NDS have some interest to use the U.S. dependency on their country's railway system to exhort more money for "keeping it safe". A little destruction now and then would help the argument.

As Lawrence of Arabia taught the Turks a while ago, logistics depending on long railway lines can be easily attacked and are very difficult to defend. This will not be the last time that such an attack occurs.

The U.S. is still learning what the Duke of Wellington (1769-1852) already recognized some 200 years ago:

"[In Afghanistan] a small army would be annihilated and a large one starved."
(source)

While the point of starving may not be reached anymore in a modern campaign it will be the high logistic costs of supporting foreign troops in Afghanistan that will eventually lead to their retreat.