When Line 12 was working properly, it cut an hour each way off the commute for María de la Luz Cobos, 57, who works in a company that sells construction materials. “When they built this line, it was a wonder,” she said. “Let’s see how long it takes to be fixed.”

Mayor Miguel Ángel Mancera raised hopes this week when he said the repair work could take less than six months. City officials are awaiting the final report of two French companies hired to diagnose what ails the Golden Line. Tests are being conducted in a laboratory in France.

“After that we will know whether the patient needs ambulatory surgery, major surgery or just treatment with pills,” said Alfredo Hernández , Mexico City’s public works secretary and the man in charge of making sure the line is fixed as soon as possible.

The consortium that built the line is led by ICA, a prominent Mexican construction firm, working with another Mexican construction firm, Grupo Carso, and the French energy and transport multinational Alstom. The group has built other Metro lines in the city without incident. CAF, the Spanish company that provided the trains, has also supplied other Metro lines without major problems.

The source of the malfunction remains unclear. About the only thing everybody agrees on is that the rails have suffered an unusually accelerated pattern of wear.

ICA and its partners noted that the specifications for the trains changed midway through the project, a point that led to widespread media coverage that the wheels did not match the rails.

But Adrián Michel, a legislator on the assembly’s investigating commission who was a high-ranking official in the former city administration, said that was unproven.