One of Southeast Asia's most successful feeder lines is testing the water farther afield with new China services.

Regional Container Lines PLC, headquartered in Thailand but operating chiefly from Singapore, said recently it is adding routes after a successful test between Hong Kong and Xiamen, a special economic zone in southeastern China."We are expanding our fleet into feeder services between Hong Kong and Shanghai after forming a joint venture with Chinese interests, and we will see a much greater volume of cargo passing through Hong Kong once new ships are delivered," said RCL manager Methawee Laowiwatwong.

The line expects China to provide an increasing share of its business when it begins operating eight new vessels. It is spending $196 million for the ships, which being built by Mitsubishi Heavy Industries in Japan and Singapore Shipbuilding & Engineering Ltd.

They will have capacities equivalent to 1,128 20-foot containers, or TEUs, and 950 TEUs. Two are due this year, the others in 1996.

"By the time all eight ships have been delivered, we will have 120,000 deadweight tons available, which puts us among the largest of the Thai fleet," Mr. Methawee said.

The additional ships are in the 12,000-dwt. class, suitable for the restricted access at Klong Toey, the river port that serves the Thai capital, Bangkok. They will also be handy for many of China's shallow-draft ports.

Mr. Methawee said the ships will also be deployed on Bangkok-Japan and Singapore-Australia runs, as well as to new destinations in northern China.

RCL bought a 20 percent stake in a cargo terminal at Xiamen last March. The island city, formerly known as Amoy, is among China's fastest-growing areas, thanks chiefly to heavy investment from Taiwan across the narrow strait.

The trial service to Shanghai farther up the mainland coast is being expanded. RCL also calls at Qingdao, Tianjin and Dalian in the north, and Guangzhou near Hong Kong in the south.

RCL made its name on the Bangkok-Singapore circuit, which still generates about 85 percent of its business. It is the Port of Singapore's second-largest user in terms of number of boxes.

The line last year began moving containers to Subic Bay, the former U.S. naval base in the Philippines now being transformed into a transshipment center and free-trade zone.