Seattle Metro, an expanding increasingly popular bus and trolley system, has negotiated a contract with its union that permits rapid expansion of rush-hour service with no increase in the number of full-time drivers.

Public transit authorities across the country, including Washington's Metro, have been reading the Seattle contract with interest because it allows the hiring of up to 700 part-time drivers - an unheard-of figure in big-city transit agreements. Transit unions, and Seattle's was no exception, traditionally oppose part-time employes.

The hiring of part-timers is seen by many transit managers as the key to solving the equation that says every extra bus - even if it is full of standees - costs more money to operate than it makes in fares. Here's why:

There are two rush hours a day in most cities and they total at most four hours in duration. Under most union contracts, including Washington's, an extra rush-hour bus, driven at the most four hours a day, requires a driver who is paid for a minimum of eight hours a day.

The Seattle Metro Council, in preparing last fall for its negotiations with the Amalgamated Transit Union Local 587, decided that part-timers were essential to the financial stability of the bus system.

The council and metro staff then mounted a well-orchestrated campaign that presold the Seattle business community on the importance of part-time drivers and the necessity of being willing to take a transit strike to get them.

Seattle Metro convinced the union it meant business and an embittered local union leadership finally accepted a contract. "It won't work," said John Senear, business agent and chief negotiator for the local. "There will be the darndest strike you've ever seen three years from now when those part-timers try to get full-time benefits."

What outsiders are trying to determine is whether Seattle Metro gave up too much for what it won.

The 1,270 existing full-time bus drivers received a pay increase of 45 cents an hour, plus a full cost of living increase every six months. Premium pay of 50 cents an hour was granted to drivers who will operate Seattle's 150 new "articulated" buses, long buses that bend in the middle (Washington is getting some too).

There were major improvements in the sick leave and vacation package, a new short-term disability payment plan, and minor improvements in medical benefits. The existing drivers were guaranteed they would never have to work as part-timers; they were guaranteed that their numbers would not fall below 900, and that there would always be 200 more full-timers than part-timers.

The part-timers would get the full hourly pay rate, would be union members, but would get only one fringe benefit: the allowance for uniforms.

Charles T. Collins, director of transit for Seattle Metro and one of the architects of the settlement, said that, nonetheless, the ability to hire part-timers "will be worth 8 per cent a year to us by 1980."

Seattle Metro, unlike Washington Metro, is on a sound financial footing. It automatically receives three-tenths of one percent sales tax and half of the motor vehicle excise tax collected in the transit district. That, plus a basic bus fare of 30 cents and suburban zone charge of 10 more cents provide a predictable source of revenue.

The taxes were voted along with a massive bus expansion plan after Seattle residents twice said no to proposed subway systems.

Even with financial security, however, Seattle's expansion plans were running into financial trouble.

Collins, one of the new breed of transit managers who comes from a financial background instead of the bus barn, took a hard-eyed look at costs and services, prepared some charts and convinced his bosses and the governing Metro Council that:

The growth potential for Seattle's bus operation was in the rush hour.

Many buses were running around during the middle of the day and at night with few passengers in them.They were running around to make work for full-time drivers who had to be paid anyway.

C. Carey Donworth, chairman of the Metro Council, put it this way: "We had three ways to move. We could hire part-time drivers for the rush hours; we could retain all full-time drivers but increase fares or subsidies, or we could abandon our expansion plans. The council quickly adopted the part-time objective."

Collins and Seattle Metro's execugether a paper that concisely presented the Metro Council's view of the part-time issue.

"The current labor agreement is not compatible with the demand for transit service," was the second sentence of the paper. With part-time drivers. Seattle Metro's long-term growth could be accommodated by 1933 withing the revenues available from existing taxes and fares. If only full-timers were employed and expansion continued, the cost of growth would outstrip revenues.

"This looks like one we can't afford to lose," a key business leader told Collins after his presentation.

As the negotiations continued to the date of contract expiration, Seattle Metro made provisions to provide alternative bus service if the drivers struck. Then it leaked the provisions.

Senear, the union leader, got the message.

"I was convinced," he said in a remarkably frank interview. "that they were going to have part-timers no matter what. Had our employes gone on strike, they would have been out for a long time then when they came back Metro still would have insisted on part-timers. We got many benefits because of it."

Senear also admitted that his union membership was too closely divided to mount an effective strike. Furthermore, it was the view of the Amalgamated Transit Union International that a strike would have been illegal; therefore it would pay no strike benefits. Senear disagreed with that view, but could not change it.

The drivers staged four sick-outs, including one on the Friday after Thanksgiving. The membership voted down a contract offer Dec. 19, but not by much.

A few sweeteners were added in face-to-face negotiations between Senear and Peterson. By Jan. 9, the contract was ratified.

The final chapter on this story cannot be written or at least another year, probably two. "We said we could make it pay, now we have to prove it," Collins said.

He wants to tap the second-job market and put the same part-time bus driver on the same rush-hour route every day. That kind of service pleases riders, and transit managers know it.

Senear and Wendell L. Duncan, the union local president did not attempt to conceal their bitterness. Collins, Duncan said, is "making the same mistake others have made. He's trying to operate the bus system as a business."