The Massachusetts Bay Transportation Authority is seeking a new ticket vendor for the Lowell and North Billerica commuter rail stations after ending its relationship with Mill City Tickets, which held the MBTA contract for nearly three years.

Until the MBTA reaches an agreement with a new vendor, passengers departing from those stations must pay their fare on board the train without incurring a surcharge, transportation authority officials said. The MBTA typically charges a $3 fee to customers who buy a ticket on board from a station where a ticket vendor is open.