This was a cool trail with great views of the lake and surrounding mountains. We were able to take the dogs with us. Several people were biking and running the trail as well.
To get here: park at the trail head or the nearby casino in Boulder city (I think it's the railroad pass) and there's a path that connects to the trail. Or start at the dam parking lot and work your way towards the trailhead. It's 4 miles one way, and then you have to take the same way back. There are two entrance gates that close at dusk (one near the trailhead and the other 3 miles in from the trailhead).
The parking area is prior to the Lake Mead fee area, so I don't think you need to pay entrance fees for this hiking trail.
What you see: five tunnels that were blasted into the mountain so that supplies could be transported from the village (gone) to the dam via the railroad. The railroad tracks are gone but the trail follows the former tracks. Some of the tunnels have started to cave in, so shotcrete was used to remediate the problem. You'll notice the rocks look different in the remediated tunnels. You'll also see some pretty spectacular views of lake mead and the surrounding mountain ranges. There's an interpretative sign that designates points of interest, such as Saddle Island.