BOSTON – The incoming head of the Massachusetts Bay Transportation Authority took a ride on the nation’s oldest – and most indebted – subway system Monday, greeting riders and calling for a major investment in public transit while acknowledging she couldn’t rule out future fare increases.

Beverly Scott, who will officially assume the post of MBTA general manager on Dec. 17, was given a three-year contract by the T’s board of directors in September after leading Atlanta’s mass transit system for five years.

Scott rode an Orange Line train from North Station to Back Bay before boarding a commuter train to South Station. Along the way, she said, she spoke to commuters who in July absorbed an average 23 percent fare increase to help close a nearly $160 million deficit that had been facing the MBTA in the current fiscal year.

With a projected $130 million deficit looming for the next fiscal year that starts July 1, Gov. Deval Patrick and other state officials have pledged to work with lawmakers on a long-term financial fix for the MBTA as well as for other regional transit systems in Massachusetts.