This is more interesting in theory, now that the weekend is over. The Baltimore Sun reports:
On the regular MARC weekend schedule, just three scheduled trains have bike cars, which allow passengers to bring full-sized, non-collapsible bikes on board the train and safely secure them in one of 23 racks. Passenger seating is available adjacent to the bike racks.
But for Friday and Saturday of July 4th weekend, MARC's Penn Line service will ran six trains northbound and six trains southbound equipped with bike cars. Meanwhile, Metro continues to ban bikes on July 4th.