The Historic Railroad Trail is a 3.7-mile out-and-back walk on a flat, wide gravel path from the Alan Bible Visitor Center to a viewpoint above the Hoover Dam parking area. It follows the old construction railway, which means a comfortable grade and five enormous tunnels cut into the cliff above Lake Mead. The tunnels are dark but not threatening — bring a torch if you want to read the construction graffiti on the walls.
The views across the lake are excellent throughout, and the trail is wheelchair- and stroller-accessible. It can be brutal in summer afternoons (no shade and reflective rock), so go early or in the cooler months. The trailhead is a few minutes east of Boulder City on the road to the dam; there is parking and a visitor centre.