Getting tickets for MARC is a fairly confusing process. There are various ways to get a ticket, all of which have their own idiosyncracies about them: (Update: MARC fixed things in 2018. See the Postpostscript.) At stations that have them, you can buy tickets from Amtrak ticket machines. This includes not only the stations that are also Amtrak stations (Union Station, New Carrollton, Penn Station, Rockville, etc.) but also some other stations that are not served by Amtrak (Camden Yards, Savage, etc.