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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 "