The Atlantic City Train Station will be closed until sometime in 2019 for major repairs upgrades. There will no longer be train service between Philadelphia or anywhere else and the Atlantic City area for some time. Once the train reopens, take a train to 30th St. Station in Philadelphia and then take a NJ Transit train to Atlantic City.
Right now you can't. Rail service is suspended between Atlantic City and Philadelphia. There is a replacement bus to Lindenwold and PATCO runs from there to Philadelphia. You would still need to get to 30th Street to get Amtrak to Baltimore
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