Forty students last night turned on an entire subway train to happiness and joy.

The students from Harvard, Radcliffe, Simmons, and B.U., rode the M.B.T.A. from Harvard to Columbia station and back, distributing free candy. flowers, and balloons, while singing "Charlie on the M.T.A." They were greeted mostly with smiles and enthusiasm.

The group met in Leverett House at 9:30 p.m., where song sheets and candy were distributed. A flower-painted leader in Disneyland T-shirt passed out instructions to "Be Spontaneous," "Make Friends," and "Let the sun rise on the M.B.T.A" The theme of the subway happening was "happiness," taken from the album. "Red Beans and Rice."

"This is the best ride I've had home in six years," an elderly commuter said to an ecstatic volunteer who had lollipop strips hung around his neck like ammunition belts.

One passenger told several girls that he would stage similar subway happenings in his home town of San Francisco. A graduate student in electrical engineering at M.I.T. continued on the train past his own stop, insisting that he be informed of the next event. Several phone numbers were received from other travelers.

The students cheered a man when he promised to participate next week, bringing with him homemade loaves of bread While most people merely accepted the free candy and goodwill one woman felt moved to give chocolate chip muffins to a group of Simmons sophomores.

Another passenger, an elderly woman, spoke for about 15 minutes with a cheerleader expressing the belief that students are the only productive force in society today.

"What you're doing is good and beautiful," she said "We're not free. We're being oppressed by our 'free' society."

The lady said she had recently been evicted from her home by the BRA.

The group almost smothered an M.B.T.A. official with flowers and candy as members surged up the stairs at the Harvard Square station. The man smiled back saying the happening was "very enjoyable." "I'd like to have it every night," he said While chewing on his tootsie roll. He added, "Now I'll get fatter."

The students held an evaluation session after the activity. A spokesman said the happening was "out of sight." This thing is going to go on," he added.