My wife and I did the old train tunnel hike up above Lake Mead a few weeks ago. It starts near the visitor's center and if you go all the way to the Dam it is about 3.5 miles, so 7 round trip, but it is an easy hike. We brought some lunch. Nice way to spend a Sunday afternoon.
There are 5 tunnels in total. The rails have been long removed.
We walked all the way down to the Dam. The new bridge is coming along. That thing is hairy!!!!!