How about MARC buys/converts one passenger car for bicycles, for use on weekends?
With the Maryland state gasoline excise tax increase, one of the transportation projects that will get funded is expansion of MARC passenger railroad service to weekends ("MARC plans to add weekend service from Baltimore to D.C." from the Baltimore Business Journal), something that was called for in the MARC Growth and Investment Plan--released before the real estate crash in 2008, and unfunded ever since.
Image: Sebuah Perjalanan.
-- Boston Globe story
-- MBTA flyer on the "Bike Coach"
While I can understand why MARC doesn't want to convert revenue-generating seats on the Penn Line during the week to non-revenue-generating bicycle parking places, accommodating bikes on weekends could increase ridership.
Bike trains in Ontario and Quebec
In Canada, during the summer, Via Rail (Canada's national rail passenger service) runs a bike car one round trip daily, with a capacity for 12 bikes in racks (otherwise bikes can be carried, disassembled, as baggage), between Toronto and Montreal, most days of the week. There is an upcharge of $20 each way.
Go Transit, the provincial railroad passenger service in Ontario runs two bike cars (total capacity = 36 bikes) on the weekends, between Toronto and Niagara Falls, for no extra charge. All regular passenger cars have two bike rack spaces.
-- Ontario Cycle Train initiative
Amtrak doesn't offer a bike car on the Capitol Limited
Note that Amtrak doesn't allow bikes between DC and Pittsburgh (and probably other places), other than as disassembled, boxed baggage, but if they did it would support tourism along the Great Allegheny Passage (GAP Call to Action) and the C&O Canal Trail.
Even though the train only does one trip each day in each direction and the times it leaves DC going north (6:05 PM) and Pittsburgh going south (4:50 AM) are pretty uncongenial.
How to approach Amtrak with a proposal to add a bike car on this train
For summer service, the way to do it would be to put a car at the end of the train (like how Amtrak deals with private railroad cars), and put it on the train in DC and take it off in Pittsburgh. They could do a car like the one that MBTA has--up to 39 bikes with 42 seats. Amtrak, obviously, would keep the passenger fare revenue, and maybe even could include an upcharge like Via Rail, if that would increase the chance they would do it.
How to fund a bike car for the Capitol Limited
I suggest that Pennsylvania (which has the great Trail Towns bicycle tourism initiative), Maryland, and West Virginia join together and purchase and convert a railroad passenger car for this purpose (DC could kick in a bit of money too).
It would be a stretch, but the National Park Service Park Roads and Transportation Program could be approached to help fund such a car/service, but typically the projects funded don't involve railroads. (While this program was repealed in the current federal transportation funding program, it still has money for some future awards and who is to say that the program won't be resuscitated?)
From the Amtrak website:
Trails & Rails - Heritage Appreciation On Board
An innovative partnership program between the National Park Service and Amtrak, the Trails & Rails program provides Amtrak passengers with educational opportunities to foster an appreciation of a selected region's natural and cultural heritage and renews the long tradition of associating railroads with National Parks.
The Capitol Limited has an onboard Trails & Rails program operating from Cumberland, Maryland to Washington, DC and is hosted by Chesapeake and Ohio Canal National Historical Park and Harpers Ferry National Historical Park.
How to take a bike via public transit to Baltimore in the meantime...
Take the subway to Greenbelt, ride the B30 bus to BWI Airport, and then take the MTA Light Rail. It ends up being something like $10 one way, depending on the rail to bus discount going towards Baltimore (you don't get that coming back).
Taking a bike to New York City or Philadelphia on inter-city buses
And on the so-called "Chinese buses" but probably not Bolt Bus and definitely not on Megabus and the more "legitimate" inter-city bus services, you can take your bike. I've seen people do it between NYC and DC. I've done it between DC and Philadelphia. I have been too afraid to do it to NYC, because I don't have a bike lock that is "New York-theft-ready."