Boulder City, about 30 miles from Las Vegas, sports the Historic Railroad Tunnel Trail, the route of the train that hauled supplies for the construction of Hoover Dam (then called Boulder Dam.) Railroad operations ceased in 1935, the tracks were finally removed in 1962, and in 1995 the trail opened.
I began at the Lake Mead National Recreation Area parking lot off Highway 93 (Great Basin Highway) -- turn onto Riverside Road and then right to the visitor center, and this lot also serves the River Mountains Loop Trail, a 35 mile paved trail. The parking area has several sign kiosks with interesting info for you. Bike a bit up the hill on the paved trail and you'll quickly see the gravel trail going off to the left.
I biked toward the dam first, about 4 miles to trail's end, which provides views such as this as you travel several hundred feet above Lake Mead. The trail is wide with no guard rails and also is a bit rough with loose gravel and dirt for a couple miles. Road bikes would not be appropriate here, but the Loop Trail mentioned above would be perfect for them.
You'll soon approach the first of the five tunnels blasted through the mountains. They are very wide, 25 feet to be exact, in order to accommodate the huge equipment that was being brought to the dam construction areas. You'll also note some foot trails on the hillsides, used before the tunnels were built.
Nice views of the marina are also apparent as you pedal...
After the fifth tunnel, you leave Lake Mead Recreation Area property and enter Bureau of Reclamation property. You are now in the security zone for the dam, and the gate is closed and locked after dark. Briefly the trail is paved since it is also a service road. Then you hit an open area with good views of the new bypass Highway 93 and the parking area for the pedestrian bridge. The trail then turns right and heads down some loose gravel to a road under the power towers where it ends at the hiker drop-off lot. The gate was closed going from here to the dam (washed out by flood) and is marked for pedestrians only when it is open.