Overnight Friday, workers cut down the old track in pieces. The next morning, a hydraulic carrier system similar to the one that moves the space shuttle eased the new section into position over Wacker Drive.

The section rested on two long, flat, red cars with 36 wheels each. Manipulating three joysticks as if he were playing a giant video game, operator Martin Moran slowly drove the span into place.

Among the spectators pressed into the corners of chain-link fence ringing the construction site or watching from a nearby bridge were Vince Gwiazda and his 10-year-old son, Anthony, from Dyer, Ind.

They watched in amazement as the machinery easily moved the 425-ton span of track.

"It's a once-in-a-lifetime thing," said Gwiazda, who added that the Wacker Drive project contained stainless steel fabricated by his company.

Construction started at 10 p.m. Friday and was slated to finish 54 hours later, in time for the Monday morning rush. In the meantime, the southbound Brown Line will stop running at Merchandise Mart. A shuttle train and buses will transport passengers to other stops downtown.

During the weekend, workers will install new support columns, girders, track and signals, according to the Chicago Transit Authority. The new design will get rid of columns that stood in the intersection.