The dinky single track Addis Ababa-Djibouti railroad ridiculed by Evelyn Waugh in his novel "Scoop" has become the object of an international dispute that has increased tensions between Ethiopia and Somalia and dealt a blow to the already fragile economy of Djibouti on the eve of its independence from France.

The line linking this sun-baked Red Sea port with the Ethiopian capital has been closed since saboteurs blew up at least three bridges near the Ethiopian town of Dire Dawa three weeks ago.

[Destruction of a fourth bridge near Dire Dawa was reported yesterday by diplomatic sources, the Associated Press said. Guerrillas also were reported to have made an attack with mortars and small arms on the provincial capital of Harar in eastern Ethiopia.]

The Marxist military government in Addis Ababa has accused Somalia, its ideological and territorial rival in the volatile Horn of Africa, of responsibility for the destruction of the bridges. Somalia, which like Ethiopia is an ally of the Soviet Union, has denied the charge. But informed sources here believe that Somali-backed nomads from the group called the Western Somali Liberation Front probably carried out the sabotage in an attempt to harass Ethiopia. The Liberation Front is trying to break Ethiopia's control over the Ogaden region, which Somalia has long claimed, and reunite it with Somalia.

It is not clear how long the rail line will be out of service nor how great an impact the rupture has had on the economies of Ethiopia and Djibouti.

At least half and probably more of Ethiopia's imports and exports have been passing through this port, on which Ethiopia has become increasingly dependent as its control slipped in its Red Sea province, Eritrea, which contains its only other ports.

Ethiopia, already plagued by insurgencies in 10 of its 14 provinces and surrounded by hostile neighbors, can ill afford any further disruption of its commercial life, and evidence that trade has been disrupted is abundant.

Two trains without locomotives sit idle in the nearly deserted railroad station, loaded with cars, trucks cement-mixers and road equipment destined for Addis Ababa. In open fields around the port, truck tires, motor-cycles, yarn, sheet steel and stationery consigned to Ethiopian recipients are piled in the broiling sun.

Diplomatic sources here say that Ethiopia has been able to move some cargo by truck convoys over a recently completed highway, but lacks the trucking capacity to haul everything it needs, especially aviation fuel. The Ethiopian vice consul here, Musa Yasin, said "Everybody knows that 50 to 60 per cent of our import and export trade goes through this port. We are trying to alleviate this problem, but we have not had time yet."

Yasin said Ethiopia has been able to divert some shipments to Assab, a port in Eritrea. If true, that might ease the situation in Ethiopia, but it would be a blow to this barren volcanic enclave, which has virtually no economic resources other than the port and the trans-shipment of goods by the railroad.

Djibouti will become independent of France on Monday. Because of historic Somali claims on the territory and Ethiopia's need to keep its arch-rival from gaining control of the port and railroad, there have been fears of armed conflict between the two over the future of the fledgling republic.

Even if it occured outside Djibouti borders a direct confrontation between Ethiopia and Somalia could have a serious impact on what will in any case be one of Africa's poorest countries. The railroad's importance is illustrated by the government's decision to keep paying the railroad workers even though service as far as the frontier has been reduced to two trains a week.

It took the French 20 years to build the line, completed in 1917. It is jointly owned by France and Ethiopia, but negotiations are under way in Paris for transfer of the French interests to Djibouti.

The line runs 484 miles to addis ababa, most of its through mountainous country in Ethiopia that is ideal for commando action and sabotage.Analysts here believe that if Somalia encourages it, the guerrillas of the Liberation Front can go on sabotaging the line almost indefinitely.

Ethiopia claims that its railway workers are working double shift without overtime to reopen the line, and that its army has secured the area well enough to insure the safety of the repair crews. But it would probably be almost impossible to protect the entire length of the line.

"Anything can happen to it, "Yasin said. "You just put three or four camels there and they can pull the rails out. The Somalis have a lot of camels."

A Western diplomat said, "If this is not going to happen again the decision will have to be made in Mogadishu." The Somalis, he said, will have to decide which they want to sustain the economy of Djibouti, whose government is expected to be friendly to Samala, or to make economic and political trouble for the Ethiopians.