If you're coming into Penn Station, it sits about a mile north of the BCC. You can take the light rail down to the Convention Center stop, which puts you at the corner of Pratt and Howard Streets--right between the Hilton and the BCC. The current fare is $1.80 each way, but that may change by the time next July comes around.
Within two or three blocks of that stop, you can find several hotels that partner with BronyCon to offer discounted rates over the weekend, including the Hilton, Marriott, Holiday Inn, and Days Inn. A few others are a bit farther out, such as the Royal Sonesta and Lord Baltimore.
A couple of fair warnings:
--As far as I know, none of these hotels offer free breakfast. I've stayed at a couple of hotels that do, including the Sleep Inn on the Fallsway and the Hampton Inn near Camden Yards, but they're not partnered with BronyCon.
--Also as far as I know, all the hotels in the area charge for parking. This one is probably a moot point in your case, since you plan to come in by train.