MBTA General Manager Phil Eng pledged Thursday to hold commuter rail operator Keolis "accountable" for staffing problems that caused a string of deflating service disruptions on the brand-new South Coast Rail extension.

Several weekend trains have been canceled on the Fall River/New Bedford Line in the month since it launched, in some cases prompting reports of stranded passengers or lengthy shuttle bus rides to replace trips.

A representative for Keolis, the private company contracted to operate the commuter rail, attributed the upheaval to problems with train crew availability.

Eng said Keolis officials communicated to the T before the extension opened that "they were ready to deliver this level of service."

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