BoltBus' and Megabus' growth in the area has been "astounding," said Joe Schwieterman, a professor at DePaul University in Chicago, who tracks trends in the bus industry.
Both firms declined to release their ridership totals in Philadelphia, citing competitive reasons.
A Megabus spokesman said that Philadelphia was one of the company's top five markets and that Megabus growth in the area was roughly in line with national trends.
BoltBus general manager David Hall said Philadelphia was his company's top market; BoltBus enjoys 5 percent to 10 percent higher occupancy in this region than other markets it serves.
Between Megabus and BoltBus, 1.5 million travelers come in and out of Philadelphia each year, according to Hall.
"Discount operators have a cool, high-tech image that people find attractive," Schwieterman said. BoltBus and Megabus passengers can book point-to-point trips online, carry a ticket digitally on their phone, and board a bus at a curb a block from 30th Street Station, rather than at a terminal.
Also, it's relatively cheap. BoltBus fares from Philadelphia to New York typically are $10 to $20. Megabus fares are more variable: Tickets purchased far in advance or for buses that depart at undesirable times can be as low as $1. Greyhound's prices for a Philadelphia-to-New York trip be $12 to more than $20.
Customers at the Megabus location near 30th Street said Monday that it was mainly price that brought them to the discount operator.
"Price is all. I'm not picky," said Brian Raczynski, 20, who takes Megabus to New York about twice a month. "Megabus is good quality for me most of the time."
That is not to say that traditional Greyhound service has suffered appreciably. Greyhound, which owns BoltBus, has coped with the competition from within by launching Greyhound Express, which markets the point-to-point, low-hassle service of discount operators to its wealthier and older clientele.
Many customers at the Greyhound terminal at 10th and Filbert Streets said they either had not heard of the discount operators or thought Greyhound was worth the price.
"You want to try something cheaper, but when you go cheaper, it's not worth it," said Carolyn Gillis, 45, of Darby Borough, who said she has been riding Greyhound her entire life.
Greyhound similarly expanded to meet a new demand when the so-called Chinatown buses were subjected to a federal crackdown in June 2012. Between 1997 and 2007, Greyhound lost 60 percent of its market share in the Northeastern United States to Chinatown buses, according to Brian Antolin, a Philadelphia transportation researcher, who has worked for BoltBus.
Greyhound seized the market for itself by launching the Asian-community based Yo! bus service, explained Hall, the BoltBus GM. While BoltBus already had a high occupancy rate in Philadelphia, Hall said, traditional Greyhound service also benefited from the newly available customer base.
After a half-dozen years of Chinatown buses' eating into Greyhound traffic, Antolin said, specialty bus services are "how [Greyhound] has been able to combat all the losses."
Schwieterman warned, however, that the recent industry changes still total increased competition for Greyhound.
"The demise of the Chinatown carriers is benefiting traditional bus carriers," he said, "but that may not be enough to offset the market share lost."
Contact Theodore Schleifer at 215-854-5607 or firstname.lastname@example.org, or follow on Twitter @teddyschleifer.