A crucial bridge providing the only access into Manhattan for all Metro-North trains traveling to and from Grand Central Terminal is due for a major upgrade, the railroad announced.

The job was advertised Friday and a construction contract will be awarded this fall for the replacement of all the original cables that lift the two, 330-foot-long main spans of the Harlem River Lift Bridge, located about 4.5 miles north of Grand Central.

“Metro-North must maintain this crucial piece of infrastructure so that it can be used by both train customers and maritime traffic. It is vital to the more than 280,000 Metro-North customers and 750 trains that use it each weekday,” said Metro-North President Howard Permut.

The work has been scheduled to do the job in the most efficient way possible to take full advantage of a six-month Coast Guard outage, during which time the bridge will not have to be opened on demand. While the cable replacement is underway, the railroad also will replace the electrical control system and most of the wiring on the bridge as well as install a new power supply system.

All the original circuit boards in the control room also will be computerized with the installation of a programmable logic control system. The elevator from the track level up to the operator’s room at the counterweight level also will be rehabilitated.

The entire two-year project, which is expected to cost $30 million, is being paid for by the Metropolitan Transportation Authority’s 2010-2014 Capital Program with help from a federal grant of $19 million.