Lillian Gash preferred not to reveal her age: "Just say I'm old." She was riding home from downtown on the Red Route.

If there were no bus, she said, "I don't think I could afford a cab. I don't drive." She lives with a daughter and her family, so "I guess I would just have to wait for one of them to come home."

Quincy Transit Lines is one of those small bus operations that many cities Quincy's size (about 42,000) gave up years ago, when the automobile became popular and privately owned transit companies went out of business.

The three-route bus system has survived here, thanks to federal and state aid, to provide mobility for students in Quincy's public schools and community college, its many elderly people and residents of the state veterans' home and county mental-health facility. Nobody pretends that Quincy Transit Lines is essential to the movement of rush-hour traffic.

Federal aid this year for Quincy Transit Lines totals $248,530. Its budget is $615,060. The federal government, therefore, is paying 40.4 percent of the cost of providing daytime independence for Lillian Gash. She pays 25 cents, the senior citizens' fare. The regular fare is 40 cents; it was last raised in 1980.

The Reagan administration proposes to forbid the use of federal aid for bus operating costs, although it is willing to continue, if much reduced, a rural aid program to buy buses.

Quincy Transit Lines is one of about 1,000 small bus systems nationwide that receive money from Section 18 of the Urban Mass Transportation Act. The Section 18 program is the price rural members of Congress have exacted in exchange for their support of big-city transit aid.

The difference is that federal aid represents at most 10 percent of the operating budgets for most big-city transits, but runs as high as 50 percent for rural systems.

The impact of Quincy Transit on the local budget is negligible. State aid, according to transit manager Elliott P. Brunier, represents $246,024, and $118,000 is collected in fares. That makes Quincy's contribution $2,506.

The first reaction to the Reagan budget by Lynn Niewohner, chairman of the city's transit advisory committee, was that the system would have to close. The city will not make up $248,000, she said.

Now her committee is looking at a number of options, including reductions in the frequency of bus runs, higher fares and an elimination of holiday and Sunday service, she said. "If we lose it all, it might shut us down," she said.

There is also hope from Springfield, where the Illinois Department of Transportation has about $5 million in Section 18 carryover money that could assist in the first years of a federal cutback.

Linda Wheeler, the state director of public transportation, said that "with such a large percentage of our Section 18 funding being used for operating purposes, recipients [like Quincy] would have to make some choices about whether to keep it or look at fares."

Quincy Deputy Mayor Lyle Nichols, who in effect is the chief administrative officer, said that transit advocates might have a tough time making their case for big-city dollars. Most of the those who ride the bus, he said, "are not politically active or astute. I suppose it does make it easier to cut. If you don't squeal, there's no grease."

The bottom line, he said, is that "nobody wants the property tax raised. The amount of revenue the city received this year was the same in dollars as the property tax revenue in 1978."

Perhaps public transit "is an anachronism" and the time may have come "to take a close look," he said. "We have a leash law. We enforce it. I expect we'd get more calls if we stopped enforcing that law than if we stopped transit."