Comment the consistently used name is just MARC as is demonstrated through a wide range of results for search site:mta.maryland.gov "marc".  The texts from the first page of results were: 
MARC Train Service;
MARC train posts delay information and On-Time-Performance...;
The Penn Line of the MARC Train Service;
MARC weekly and monthly tickets ...;
MARC now offers weekend service ...;
Penn Line trains will have the MARC Bike Car in the train;
MTA strives to make it as convenient as possible for MARC riders to stay informed and up-to-date;
The MARC Riders Advisory Council  ...;
MARC Train and Commuter Bus Tickets ...;
The new parking garage at the Savage MARC station
site:mta.maryland.gov "marc" gets "About 7,930 results"
site:mta.maryland.gov "marc train" gets "About 3,320 results" many of these results don't come from text but from a repeated link reading "MARC Train"
-site:mta.maryland.gov -Wikipedia "marc" AND (penn line" OR "camden line" OR "Baltimore Penn Station") gives many results other than "MARC Train"
Suggest  
Kaye