Officials said that more work was needed than previously imagined, with a number of parts to be replaced.
A first forecast that the RER B line between central Paris and the airport would be up and running early Wednesday was first changed to mid-morning and later midday and then 4.00pm, as work progressed slowly.
RER B is Europe's second-busiest rail line, carrying 900,000 passengers a day.
The breakdown of the service came as traffic restrictions were enforced in Paris because of high pollution levels.
The restrictions on cars according to their registration numbers continued on Wednesday.
The SNCF has laid on two TGV high-speed trains and buses to shuttle passengers to the airport from the Aulnay-sous-Bois station.