CSX says it has drained two of the five cars still derailed Friday over the Schuylkill River. Each contains 30,000 gallons of crude.
The accident Monday follows a series of derailments involving Bakken Shale crude from North Dakota - including one that exploded in Canada, killing 47 people.
U.S. government officials have been pressing the industry to address safety concerns.
CSX spokeswoman Melanie Cost says the wintry weather has slowed work at the Philadelphia site, which is next to the Schuylkill Expressway and near the University of Pennsylvania.
She says the crews are moving slowly to ensure the transfer is done in a "safe and environmentally responsible manner."