Two transportation agencies have launched a campaign to encourage travelers to take the train to Newark’s airport.   Officials of NJ Transit and the Port Authority of New York & New Jersey announced new promotional discounts and outreach programs on Thursday.   Beginning today, passengers traveling between the rail station at Newark Liberty International Airport and New York’s Penn Station will be able to purchase tickets for $12.50 each way.   NJ Transit also announced a Family SuperSaver fare, which allows up to two children, age 11 and under, to ride to the airport free with each adult. The fare will be offered through Jan. 2.