Workers returned Friday to a South Philadelphia factory after walking off the job on Thursday to protest a lack of water and air conditioning.

About 50 to 60 employees left the Hyundai-Rotem USA Corp. factory at noon Thursday, 31/2 hours before the end of their shift, workers said. The factory is assembling Silverliner V rail cars for SEPTA, and workers have been involved in a long and acrimonious effort to reach agreement with the company on a union contract.

Workers said they complained to managers on Thursday morning about a lack of cold water and fans, and about a broken air conditioner in the employee lunchroom on a day when outdoor temperatures approached 100 degrees.

The workers, mostly African Americans and Hispanics, said South Korean workers had a functioning air conditioner in the trailer where they eat lunch apart from the American workers.

Hyundai-Rotem, a subsidiary of South Korean automaker Hyundai Motor Group, has brought increasing numbers of workers from its South Korean headquarters to South Philadelphia to try to speed up work on the rail cars. Delivery of the new rail cars to SEPTA has fallen more than a year behind schedule because of continuing material shortages, design flaws, production problems, and workmanship errors.

There have been culture clashes and poor communication between South Korean managers and American workers, prompting complaints to the National Labor Relations Board about abusive treatment. The workers last year voted to join the Transport Workers Union and have been negotiating since to reach agreement on a contract.

The company did not respond Friday to inquiries about the workers' walkout.

Contact staff writer Paul Nussbaum at 215-854-4587 or pnussbaum@phillynews.com.