You don't need lights through the tunnels during the day as there's solar powered lighting.
I first rode this route after reading about the completion of the tunnels, soon after the work had been done. What I didn't realize was that the lights are turned off at dusk.
It would have been a tough day's ride from where I live so I used the car to get to Wyedale, the nearest parking point to the Monsal Trail at the Buxton end. It was a dull November afternoon, and I had a pleasant ride with the downhill trend in my favour towards Bakewell. Even in November the route was busy with walkers and cyclists, some of them families with kids. Most of them seemed to be enjoying being on the trail. The surface was good, manageable on a road bike with no difficulty I would think. I was using my Claud Butler Dalesman and I had no issues. It was only when I got into conversation with another cyclist not far from the Bakewell end that I learned that the lights in the tunnels went off at dusk. There are six tunnels on this trail, and only the longer ones are lit, as there is enough natural light from each end to see your way through the shorter ones. I might have dawdled less if I'd known that the lights went off after dark.
Dusk came very quickly once the sun went down behind the surrounding hills and I was surprised to find how pitch black it suddenly became. I continued beyond Bakewell as I wanted to see the end of the trail. I came across it suddenly, almost running into the barrier across the fenced off Coombs Road Viaduct. This was about a quarter mile beyond Bakewell. I had only brought one headlight with me, my good Cateye and now wished that I'd brought a back up. As it was, it was barely adequate to see where I was going with the lack of ambient light on the remote trail. I retraced the route and it was a different world. I had to use my peripheral vision to detect shapes in the darkness, not helped by my glasses being sweaty. I passed Bakewell station, then Hassop station, both in darkness.
Eventually I came to the entrance to Headstone tunnel, well marked out with lines of white paint. A cold breeze wafted from within. I took a deep breath and plunged inside. I had to slow right down, as the blackness ahead was almost tangible. Beyond the gleam of my front light the damp walls and roof glistened faintly, but otherwise it was like riding into black velvet. I knew there was a shallow drainage ditch each side against the walls, so did my best to stay in the middle. Fortunately there was a white line each side of the surfaced area, which showed up dimly in my light. The tunnel curved gently and soon the shrinking spot of grey that was the entrance disappeared from my mirror. There was no sign of the exit. I stopped, and the scrunching of my tyres disappeared. It was absolutely quiet, save for a distant drip and trickle of water. I had an irrational impulse to switch off my light just to see what it was like. Suppose it didn't switch on again? Suppose I clumsily knocked it off the bars and couldn't find it it in the dark? I gripped it with my left hand and turned it off with my right.
Total blackness. I felt far underground. I felt the balance between millions of tons of rock above doing battle with this man made tube of rock and brick, the one trying to crush, the other resisting. How many men had died, blasting and muscularly digging, to build this symbol of human mastery over a natural obstacle? I found it hard to breathe. A cool gust fanned my face, reminding me that I was sweating. Enough of this. I was feeling uncomfortable. The light flickered and came on. I found I could see a little better after being exposed to total darkness. I felt a powerful need to be out in the air again. This tunnel was about a kilometre long, and I must have been about halfway through. I heard sounds ahead, then saw lights, and a group of joggers thumped past. They said Hi, I said Hi, and I felt much better. A grey disc appeared far ahead, got bigger, and soon I was out in the air. A couple of walkers with head lights came past. There was still some traffic on this trail even after dark.
A longish stretch in the dark before I entered Cressbrook tunnel which was fractionally shorter than the last one, followed by Litton tunnel immediately afterwards. Here I detected a distant cycle light behind gradually gaining until he passed me and turned off at Millers Dale station.
From then on it was a fairly easy trundle through the short Chee Tor 1 and 2 tunnels then Rusher Cutting tunnel. A clear open air stretch to the road access by Blackwell Mill and back to the car at Wyedale car park.
Mrs Twiddler and I have walked parts of it since, and have done the whole length and back on folding bikes but for me, that first experience on my own in the dark remains my most intense memory of the Monsal Trail. I would recommend a visit to this cycle trail for anyone (preferably in daylight) once Coronavirus restrictions are eased and the facilities are all open again. Probably easier to start at the Bakewell end, as it is trending downhill on the way back.