MEXICO CITY (AP) — Mexico City's subway system has closed about half of its newest line because of design problems, forcing hundreds of thousands of riders to use buses instead.

The city's aging subway has struggled with numerous problems, including overcrowding, mechanical problems, a very slow expansion.

The newest line, inaugurated in 2012, was the first opened in about a decade.

When authorities this week discovered design problems causing excessive wear on the rails, about half of Line 12 was shut down and hundreds of buses were brought in to transport commuters.

Metro officials said Thursday that 435,000 passengers per day had to take buses because of the partial line closure.