Last year, nearly 50 million passengers -- many of them swearing -- pushed through busy O'Hare International Airport while fewer than 100,000 walked easily through Midway Airport located just across town.

Once the world's busiest airport with 20 million passengers a year, Midway fell on hard times when the major airlines pulled out to go to the newer, bigger O'Hare in 1973. Because it has been nearly a ghost-town ever since, it was not surprising at all to hear a man in the small bar at Midway a couple of weeks ago exclaim to his companions, "This is an airport," when the sound of a descending airplane intruded on Midway's normal silence. The plane was one of just three daily scheduled flights that broke the quiet here.

But things may be changing now with the birth last week of a brand-new airline named after the airport that will serve as the center of its activities. Aided by deregulation and a surrounding community that is uncharacteristically enthusiastic about the sound of roaring jets above. Midway Airlines may succeed in putting Midway Airport back on travelers' intineraries.

The time appears right for it: Many business and other travelers who dread the delays and congestion that have become practically synonymous with O'Hare, and others who will be attracted by the unrestricted cut-rate air fares that the new Midway Airlines offers on all its flights, may find Midway a godsend.

Passengers will find it takes less time to get to Midway from downtown Chicago by cab or bus than to O'Hare, less time to park a car in the 1,300-space, nearly empty parking lot in front of the terminal, less time to buy a ticket with the airline's simplified ticketing system, less time to get to the gates which are nearly adjacent to Midway Airlines ticketing counters, less time waiting on the ground in the plane while the pilot waits his or her turn to take off and less time for baggage retrieval.

And where possible, Midway plans to fly to other convenient in-town airports such as Cleveland's Burke Lakefront Airport, located just minutes -- and even within walking distance -- of downtown Cleveland.

"If we do our jobs, once people travel with us and see how easy it is compared with O'Hare, word will spread," Irving T. Tague, president of the new Midway, said confidently.

Midway initially will offer 28 daily flights -- 184 a week -- between Chicago and Cleveland, Detroit and Kansas City on 83-passenger DC9 aircraft. Midway's regular coach fare on weekday flights is about 25 percent lower than its competitors, and its weekend and some offpeak weekday fares run 50 percent lower. For instance, it will cost $49 to fly between Chicago and Cleveland on weekdays during peak hours and $33 during off-peak hours and the weekends. The established airlines charge between $63 and $67 for the same service.

Even before Midway's service began on Thursday, word had spread so fast that its initial reservations staff of 15 could not answer the calls that began to pour in each day. For instance, last Monday -- the day before Midway's inaugural festivities -- 1,599 calls were answered and 1,074 reservations were made, but 3,453 calls were lost, a computerized counter indicated. The number of calls, and lost calls, continued to build, and Midway decided to double its reservations staff and increase the number of phone lines.

"This is our problem," a beaming Kenneth Carlson, vice president of marketing, said during a tour of Midway's facilities, including the room with two long tables of phones that serves as the reservations center. "It's one Irv and I always hoped to have."

One significant attention-getter was Midway's announcement of a promotional "pocket change" fare for the first weekend -- when passengers, on a first-come, first-served basis -- would be able to fly literally for "pocket change." The promotion, which reduces the $33 weekend fare to Cleveland, for instance, to 33 cents, and so one, swamped Midway with 17,000 calls.

But more important, on Wednesday the level of daily bookings reached 1,200 -- the number Tague and Carlson believe they need daily to make Midway a success. On Tuesday, there were 1,600 bookings. Four flights were sold out for Friday; a total of 31 flights were sold out for around Thanksgiving. Tague jokes that Carlson scheduled a "brilliant marketing coup" by arranging the Thanksgiving and Christmas holidays -- periods already solidly booked on other airlines -- during Midway's first two months of operations. Midway expects to carry 700,000 passengers the first year, and double that the second.

If enough people travel on Midway in the next two months and are happy about it, Midway's officials beleive the airline will be able to draw on its success to sustain loads through the winter -- when travel traditionally is slow and weather often bad -- not to mention the recession that many beleive may cut into travel budgets.

"If I have any concern at all, it's with the first quarter," Tague said. But some "Tague economic theory" gives him hope: "In the last recession, the short-haul carriers did better than they had done before and better than the larger carriers," he said.

Barely off the ground, Midway already has plans to expand -- very likely to Washington, D.C., and Minneapolis/St. Paul -- when two more DC9s are delivered in the spring. The first five planes are leased, but Midway has placed orders with McDonnell Douglas for five new 120-passenger DC9s for delivery in 1982 and after. So far, Midway has authority to fly to another 26 cities from Midway Airport.

The first airline to be created since the Airline Deregulation Act and the first in U.S. history to start passenger operations as an all-jet interstate carrier. Midway was three very long years in the making. After the Civil Aeronautics Board authoritywas granted, it took Tague and Carlson a full year to raise the $5.7 million needed to start operations from 16 different sources, mostly venture capital firms. None of the money came from Chicago, despite the city's and state's interest in revitalizing the airport and the depressed environs surrounding it. (Midway's initial payroll is $3.2 million annually, and many of the 200 hires came from neighborhoods around the airport.)

Besides using convenient satellite airports when possible and offering low fares. Midway has also come on board with an innovative ticketing program. Passengers can buy a book of 10 tickets that are good for a year, protecting them against fare boosts and giving frequent travelers the pleasure of bypassing the ticket counter.

Midway is offering the same books of 10 tickets for the price of nine to travel agents, giving them a 10 percent commission on their sales.The tickets are refundable, but Midway is insisting that the agents pay for the tickets up front. Initially the agents resisted but interest in Midway from their customers has made some of the larger agencies decide to buy the books and offer Midway's tickets to their customers.

The first couple of days of operation went off without a hitch -- "It's working beautifully," Carlson crowed -- but there are problems that have to be worked out. For instance, not having any operating experience, Midway declined initially to "overbook." that is, give passengers more confirmed reservations than there are seats on a flight to compensate for the "no shows:" in the first few days, between 10 percent and 20 percent of the passengers with confirmed reservations failed to show.After it collects some accurate statistics, Midway probably will begin overbooking as other airlines do.

(There is another problem Midway's officals will have to resolve: During the inaugural ceremonies last week, a group of the new flight attendants were complaining in the restroom that the newly issued shoes hurt their feet.)

Because of its simple route, fare, ticketing and corporate structure -- its "corporate offices" are plain rooms in the airport off the concourse near their gates -- Midway won't need as many passengers as others do to make money. It estimates that it will break even when just 47 percent of its seats are filled systemwide with its mix of 60 percent regular coach, 40 percent off-peak economy. Right now, 65 percent of its advance reservations are for the off-peak "pleasure" fares and 35 percent are booked for its normal coach fares.