Zambian President Kenneth Kaunda's decision to again start using Rhodesia's railway was good news for the beleaguered Salisbury government. But it is also a major coup for South Africa, whose railway will handle Zambian goods for an important leg of their journey to and from ports on the Indian Ocean.

The controversial Zambian decision could not have come at a more propitious time for South Africa, which is facing renewed calls for economic sanctions at the United Nations because of its rejection of a U.N. election plan for Namibia (Southwest Africa).

Kaunda's decision has also given a boost to Pretoria's diplomatic efforts to reduce its growing isolation from other African countries, as a group of Zambian railway officials arrived in Johannesburg this past weekend for discussions with their white counter-parts. A South African Foreign Ministry official said he could not recall a visit of this semi-official nature between the two countries since the economically hardpressed Zambian government closed its rail link with Rhodesia and South Africa in February 1973.

The talks between Zambian and South African railway officials, which began secretly in Botswana last week, center on tariffs and other details of transport of Zambian goods through South Africa between Mozambique and Botswana. Tons of American-supplied fertilizer for Zambia are already being transported from the Mozambi-can port of Maputo through South Africa. In the future, Zambian goods may be shipped directly to the South African port of East London, according to some reports.

The renewed rail link also butresses South Africa's contention that economic considerations of the black-ruled African states will prevail over the "ideological" ones that aim to bring down South Afirca's racially discriminatory political system of apartheid.

Zambia's situation "is clear evidence of the harm which sections against South Africa would do to all nearby states," the government-run broadcasting station commented. If sanctions are imposed on South Africa, then it cannot help black-ruled states in southern Africa, the white minority government argues.

"The five foreign ministers expected here later this week, will have to adjust their arguments" following the Zambian decision, the radio station said.

U.S. Secretary of State Cyrus Vance is to join the foreign ministers of France, Britain, West Germany and Canada in South Africa for top-level discussions with Pretoria on the Namibian issue. The foreign ministers are expected to discuss their veto at the United Nations if a sanctions vote on South Africa comes up. They could possibly hint to Pretoria that they will not use it if South Africa refuses to cooperate with the U.N. plan on the future independence of Namibia.

Mozambique already accepts South African assistance in running its railways and harbors. Zaire has never stopped exporting its cooper through South African ports.And recently, the Marxist government of Angola asked the South African diamond mining company, De Beers Consolidated Mines Ltd., to send a team to Angola to help reorganize its trouble diamond industry.

Zambia's latest action, in the words of the commentator on the government radio station, is an example of the "priority which sooner or later must be given to economic over ideological considerations."

South Africa must also welcome the reported split over Kaunda's decision among the so-called front-line states that are involved in finding a solution to the Rhodesian conflict. According to reports from Lusaka in the Johannesburg Star newspaper, presidents Julius Nyrere of Tanzania and Samora Machel of Mozambique pressed Kaunda in vain at a special meeting on Saturday to reverse his decision because it wrecked the front-line states' solidarity against the Rhodesian government.

The South African government radio noted that Kaunda's decision "opened a rift between militant (Tanzania and Mozambique) and moderate (Zambia) states."

Finally, Zambia's renewed dependence on the Rhodesian railway, and ultimately on the South African rail system, gives South Africa increased leverage. Over Zambia and the Rhodesian guerrilla leader based there, Joshua Nkomo, in any future discussions on the Rhodesian conflict.

James MacManus of The Guardian reported from Luaka, Zambia:

President Kaunda's refusal to reconsider the opening of Zambia's trade routes through Rhodesia and South Africa has left relations among the five front-line nations in a state of disarray. On Saturday presidents Julius Nyerere of Tanzania and Samora Machel of Mozambique arrived here on a surprise visit in an effort to persuade Kaunda to rethink his decision to switch the country's trade to the southern route.

According to reliable sources here, the Zambian leader politely but firmly made it clear that his decision was irreversible.

Sources in Tanzania say that Nyerere had been irritated by Kaunda's assertion at a Friday press conference when he announced the border decision that all the front-line states had blacked his move.