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A seven-month project designed to eliminate slow zones by replacing 6,000 deteriorated rail ties means that the Purple Line will run more slowly on nights and weekends for most of the remainder of 2013. After the project is completed, though, the train should move much faster, said to CTA spokesperson Bill Mooney.

Construction will take place Sunday through Thursday between 9 p.m. and 4:30 a.m., and all day on weekends. Some work can be quite noisy, warned Mr. Mooney, thought the CTA plans for the loudest work to take place between 9 p.m. and midnight.

The last major repair to the purple line was in 1976, and railroad ties have a life expectancy of about 25 years. The repairs, said Mr. Mooney, are overdue.

A specially designed machine will extract the deteriorated ties and insert a new tie. Another machine will then spike the actual rail to the tie. Yet a third machine will tamp down the rock and gravel ballast for a quieter, smoother ride.

Alderman Ann Rainey, 8th Ward, asked about progress reports. Matt Swentkofske, the City’s intergovernmental relations coordinator, said that the City’s website would post regular progress reports, and that anyone with questions should call 311.