Judging by a couple of videos posted here, including one by Gray Hughes on YouTube, it would take approximately 7 minutes to walk from the parking are to the bridge. The bridge itself could be crossed by a healthy adult male in under 4 minutes, byt my simple math, as the bridge is 850 feet long. The girls were taking their time, plus we don't know if there were geocaches or other stuff they may be looking for along the way.
I think at this point we need to keep in mind the actual trail ends at the bridge, and the bridge itself is still CSX (private) property. Be that as it may, I figure they were almost at the SE end of the bridge in under 25 minutes from being dropped off, probably hanging out up there, taking pics and video. Could have taken 30 minutes, I think that's about our max time from drop off to the end of the bridge.
Either before then or around then, I'm thinking 2:15-ish, is when BG appears.