The plan was to remove an aging 300-ton bridge deck from over the Baltimore-Washington Parkway in one piece and install a replacement — all in a single weekend.

But things went wrong Saturday, and after it was freed from its moorings, the massive deck shifted position. It became necessary to halt the delicate removal process and then to reverse it.

On Saturday evening, with the help of four giant cranes, the overpass deck on West Nursery Road in Anne Arundel County was nudged back into the position it had occupied since 1948.

Early Sunday morning the state highway administration said it had reopened the northbound lanes of the parkway, which had been shut down for hours. Inspectors had scrutinized the bridge structure, which is in the Linthicum area, and determined that it was safe to travel beneath it, the SHA said.

The plan had been to unbolt and cart away the old deck Friday night and Saturday morning. The new deck, already prepared, was to be installed Saturday night and Sunday morning.

A repair or replacement job that might take months was to be done in hours, said SHA spokesman Dave Buck.

The key was to be the Self Propelled Modular Transporter, which is a computer-controlled platform vehicle that can move bridges weighing up to several thousand tons with precision to within a fraction of an inch.

But at 12:45 a.m. Saturday, as crews placed the old bridge deck onto the transporter, the load slid about three feet out of line, Buck said.

Fortunately it remained intact. The decision was made to return it to its original position. By 7 p.m., Saturday, the deck had been reseated, Buck said. The next steps were to restore bridge approaches and inspect the bridge.

Then, Buck said, authorities must determine what went wrong and what to do about replacing the deck over the southbound lanes.

Although the northbound parkway lanes had reopened, authorities said, as of 1 a.m.Sunday West Nursery Road remained closed.