Going through Pens. Avenue I think if memory serves correct, there is a utility tunnel that is next to a brotherhood of steel member at a barricade thing. This Utility Tunnel is what your looking for, getting there is fairly simple and if you don't like ferral ghouls just get the Ghoul Mask or bring Fawkes along and problem solved. However, getting to Pens. Avenue is probably what you should be asking. Most Metros connect to each other, just add a map marker to the Pens. Ave. area and see which metro that you have found lights up to get there.