New York (CNN) -- Superstorm Sandy has weakened, yet her presence will be felt in the days and weeks to come as transportation systems assess impacts and try to resume schedules.
Much of Tuesday's air and rail service has been canceled, and millions of public transit commuters are without service. Here's what to expect in many of the affected areas:
New York City's massive public transit network was crippled overnight.
Seven subway tunnels under the East River flooded in the course of the storm, the Metro-North Railroad lost power in sections of its lines and the Long Island Rail Road sustained flooding in one East River tunnel and evacuated its West Side Yards, according to the statement. The Hugh L. Carey Tunnel flooded "from end to end," the Queens Midtown Tunnel also was closed because of flooding, and six bus garages were disabled.
Lhota said MTA employees will restore service as quickly as possible "to help bring New York back to normal."
Transit officials in Philadelphia said they are optimistic that city and suburban services will be up and running sometime Tuesday but could not give an exact time.
Southeastern Pennsylvania's regional rail commuter lines present a bigger challenge, said SEPTA spokeswoman Jerri Williams. Crews were out overnight and were walking the lines Tuesday assessing damage with the hopes of resuming commuter rail service Wednesday morning.
"But we already know it's going to be a challenge. We already know there's going to be disruptions and delays in the morning," Williams said.
In Boston, most transit service was set to resume Tuesday morning, with some delays, according to the Massachusetts Bay Transportation Authority.
Amtrak extended Northeast Corridor cancellations through Tuesday. Bus routes connected to those trains were also canceled. Trains coming to and from Canada or to and from the South will still operate, but they will stop short of the storm-affected states. Consult Amtrak's website for more details.
Thousands of flights canceled
There have already been more than 15,000 flight cancellations since Sunday as a result of Sandy, according to flight tracking site FlightAware.com.
That number is expected to rise, since FlightAware.com reports that most airports in the New York City area are closed and haven't announced reopening times.
"It is unlikely there will be scheduled flight operations to/from NYC today and some airlines have begun canceling flights on Wednesday," according to a FlightAware.com statement.
There have been 6,047 flights canceled for Tuesday, as of 8:50 a.m., FlightAware figures show. Philadelphia is leading the way with 1,085 cancellations followed by the New York-area airports with nearly 1,000 each.
Airlines have already canceled 635 flights for Wednesday. More cancellations are expected Tuesday and Wednesday as airport staffs assess damage.
American Airlines, JetBlue, Southwest Airlines (including its AirTran subsidiary), and US Airways announced cancellations Monday of all Tuesday operations to many Northeast locations, including the Philadelphia, Washington, Boston and New York City-area airports. United Airlines has canceled flights at the New York City airports and only expects to restart operations in the Washington area Tuesday night.
Delta Air Lines said Monday that "limited flying is expected to resume Tuesday afternoon at Delta's LaGuardia and JFK hubs with a full restart targeted for Wednesday, subject to conditions." An airline spokesman says Delta also plans to restart operations Tuesday morning at other Northeast airports, including the Boston and Washington-area airports, conditions permitting.
Most carriers will allow affected passengers to change their itineraries without penalty. Check the major airlines' advisories here: American Airlines, Delta, United, US Airways, AirTran, JetBlue, Southwest.
Middle Eastern, European and Asian airlines also grounded flights in and out of the United States' Eastern Seaboard as Sandy approached.
The good news is that the storm is happening during a slow travel season and airlines canceled many of the flights in advance, minimizing the number of stranded passengers, according to Farecompare.com Chief Executive Rick Seaney.
"Barring significant airport damage, flight patterns should be back to normal by the end of the weekend," Seaney said.
CNN's Aaron Smith and Greg Botelho contributed to this report.