For the city pairs below, I largely used the equation of flight time plus three hours to account for commuting to and from downtown areas on both ends to come up with true air transit time when compared with train travel.

The tipping point seems to be just shy of 300 miles, making long distances more efficient by air, especially if your time is limited. Flying from New York to Toronto, about 500 miles apart on the ground, for example, takes about 90 minutes and even if you generously add four hours commuting, you’re still better off flying versus taking a 12-hour-and-30-minute train.

Boston to New York City

Given the proximity of Boston Logan International Airport to downtown, and its many quick public transit options for travel there — running roughly 20 to 30 minutes from downtown — I adjusted the equation to flight time plus two and a half hours. I found midweek flights between Logan and New York City’s La Guardia Airport from $165 round trip on American Airlines and Delta Air Lines, with a flight time of about one and a half hours. Adding two and a half hours came to four hours true transit time.

The Northeast Regional train takes between four and five hours for $110, versus the Acela, which takes three hours and 50 minutes, and costs $214, both fares round trip. Hipmunk found the average savings for train travel is $110. Acela was about $50 more expensive for about the same travel time, while the Northeast Regional matched travel time and saved $55. It’s no wonder the Northeast Corridor is Amtrak’s busiest.

Boston to Washington

Boston to Washington, about 440 miles apart, is another story. If you allow 30 minutes to get to the airport, 60 minutes preflight, 100 minutes in the air and 60 minutes on the other side (conservatively; Washington National Airport is about 20 minutes from downtown via Metro train), that’s just over four hours. The Acela takes seven hours; the Northeast Regional is closer to eight hours. The three to four hours difference favors flying. Could price make a difference?

No. I found round-trip flights on JetBlue at $117 between Boston Logan and Washington National and round-trip train tickets at $376 (Acela) and $267 (Northeast Regional). Flights were cheaper and quicker.

New York City to Washington

Hipmunk’s data showed median New York City to Washington National round trips by plane at $292. I found cheaper nonstops from $257 on American and Delta. Flights take about 75 minutes, putting true transit time at four hours and 15 minutes.