Just an addendum:
I don't know if you plan on traveling from Penn to Union on the weekday or weekend, but the MARC doesn't run on the weekends. You would have to take the AMTRAK, which is a bit more expensive (~15-30$, depending on the time), and the weekday MARC is ~$4.00.
As for the shuttle from Hopkins, I'm not sure if you're going from the undergraduate campus or the medical school campus, but the stops are either at Shriver Hall or in front of the Johns Hopkins Bloomberg School of Public Health. They will both take you to Penn Station, right in front. Be sure to have your license with you because depending on the driver, sometimes they get picky about having a Hopkins ID to ride the shuttle. If you say you're an interviewee, I'm sure that they can comply.
Hope this helps!