The Carter administration yesterday announced a package of grants, loans and new projects aimed at improving public transportation in isolated rural communities.

The plan, disclosed amid much fanfare by three Cabinet secretaries in ceremonies at the White House, is part of President Carter's forthcoming "comprehensive rural development policy" that will be introduced later this month, according to presidental aide Jack Watson.

The transportation segment of that comprehensive plan contains some new programs, but has the overall effect of consolidating the already existing 114 federal programs currently providing transportation assistance to rural areas.

In the transportation plan, the General Services Asministration and the Defense Department have agreed to give "several hundred excess or surplus vehicles" to rural areas, while 1,500 Comprehensive Employment and Training Act workers will be trained as drivers, mechanics and dispatchers in 13 demonstration states.

Also in the plan was an agreement among several aviation and business regulatory agencies to target more than $200 million in grants, loans and loan guarantees to commuter airlines serving rural areas and to small community airports to upgrade their facilities.

A similar grant and loan package was included for the rehabilitation and repair of abandoned or seldom used railroad branch lines, at an estimated cost of $30,000 per mile of track.

The final portion of the rural transportation plan is series of federal incentives for van pooling and ride sharing. The incentives listed included an Energy Department guarantee of 100 percent of needed gasoline for all registered van pools during shortages, and Transportation Department efforts to find "reasonably priced" insurance for van pools.

Administration officials, at a White House press briefing before the official announcement, were reluctant to put a final price tag on the new rural transportation plan, since most of the money was being redirected from existing programs.

"It's not new money. It's money that's being refocused based on these new priorities," said Larry Gilson, the associate assistanto the president who conducted the briefing.