BOSTON--The Massachusetts Bay Transportation Authority plans to expand the network of commuter rail lines in and out of Boston by the year 2000, carrying as many as 80 percent more riders than now.

Transit planners say the expansions could increase total system ridership involving South Station from the current level of 29,000 a day to 52,109 in the year 2000. Ridership at North Station is expected to increase by 38 percent, to 15,737 daily by the year 2000.

Besides the planned restoration in 1993 of the Old Colony commuter line from South Station through Braintree, other MBTA expansion plans include:

.Extending the Framingham line to Worcester through Ashland, Westboro, Grafton and Millbury.

.A new branch off the Needham line, running from Needham Junction to Millis through Dover and Medfield.

.A new station, already under construction, at Interstate 495 on the end of the Franklin line.

.Extending the Stoughton line to Taunton through Easton and Raynham.

.Extending the Ipswich line to Newburyport with stops in Rowley and Newbury.

MBTA General Manager James F. O'Leary told the Boston Sunday Globe that transit officials want to proceed carefully with the expansion.

"I want to make sure we have sufficient capacity to meet the demand in Greater Boston before we extend out to [Interstate] 495," O'Leary said.

The MBTA currently operates 10 commuter rail lines, five each out of North and South Stations, and a shuttle from Readville to South Station through the city's Dorchester section.

With ridership on commuter rail lines having increased 49 percent between 1983 and last fall, the MBTA's rail fleet is scheduled to grow to 248 cars by July, a 48.5 percent increase since last month.

Also, the MBTA expects to begin ordering this spring its first bi-level cars. with will seat 160 riders on two levels, compared with 110 for standard single-level cars.

The planned service extension also mean the MBTA will be forced to look for more train storage space. And South Station, where all trains now use only tracks to reach the station's 11 platforms, will have to be renovated.

Planners working on the Old Colony line project say locations have been fixed for 16 of the 22 planned stations. The planners hope to provide more than 8000 parking spaces for the 13,000-rider system.

In addition. Old Colony officials said they hope to complete an important environmental report by February, clearing the way to build a new bridge burned down shortly after service on the line was abandoned in 1959 following the opening of the Southeast Expressway.

State officials say they are counting on the federal government to pay half of the expected $391 million cost of restoring the 81-mile rail system.

However, time ran out on the 1987 legislative session before the Senate could pass a $3.1 billion transportation bond issue, which could have included funds for the Old Colony project.

Also, Alfred A. DelliBovi, administrator of the federal Urban Mass Transportation Administration, has backed off from a commitment made by his predecessor. The former chief, Ralph Stanley, had said the project would be given top consideration for funding if the state paid 50 percent of the cost.