A security officer stands guard as passengers line up for a security check during the morning rush hour at Tiantongyuan North Station in Beijing. (Jason Lee/Reuters)

The Beijing Subway may be the busiest in the world, with 16 lines completing more than 10 million passenger rides a day. That's an impressive feat, but it also represents a serious security problem – especially in the wake of a bomb attack at a train station in Urumqi last month that left three dead and a knife attack the month before in Kunming, Yunnan province, in which 29 people were slashed to death.

Because of that risk, some Beijing residents are facing a serious problem with their commutes, as the photographs here show.

Passengers wait for security checks during rush hour at Tiantongyuan North Station in Beijing. All passengers at this station are required to undergo such screenings. (Jason Lee/Reuters)

Tiantongyuan North is one of nine train stations, mostly near Tiananmen Square in central Beijing, that require all passengers to go through a security check before entering. The security checks began in January, building on previous checks on passengers' luggage that were put in place during the 2008 Beijing Olympics.

According to the Nanfang, the already strict security was "dramatically tightened" recently, resulting in the scenes you see here.

It may seem like an extreme measure, but it's understandable. Although the train station attacks occurred far from Beijing, the Chinese capital has been touched by terrorism before: Last year, members of the Uighur ethnic minority rammed through crowds in Tiananmen Square, killing themselves and two tourists.

The long lines aren't the only sign of a security push at Beijing's subways. According to CRIEnglish, police officers armed with guns were recently stationed at five major subway stops, including Xidan, Dongdan, Tiananmen East, Tiananmen West and Wangfujing. It is reportedly the first time that regular Chinese cops have been armed with guns.

The security lines are long at Tiantongyuan North Station in Beijing. (Jason Lee/Reuters)

As my colleague Ishaan Tharoor wrote recently, there a lot of reasons to suspect that China's Uighur terrorism problem may be worsening. And while a growing Islamist militancy in the region may be a serious factor, the government's heavy-handed response to ethnic problems in the Uighur homeland of Xinjiang hasn't helped.

Of course, you have to wonder if the mass of people outside Beijing's train stations are themselves a prime target for terrorists. “I am afraid that not many could safely escape if terrorists suddenly draw knives within the crowd,” one blogger quoted by the South China Morning Post said.

Tiantongyuan North is one of nine stations where security checks have been ramped up in the wake of train station attacks. in (Jason Lee/Reuters)

For some Beijing commuters, however, the new lines may he just another hassle in an already difficult commute. This video shows how bad rush hour can get at Beijing's Xierqi subway station. It was shot in July, before the latest security measures were implemented: