I'd say that about 50% or more of my travel is solo, and its going to become increasingly so. All of my rail travel most certainly is, with only one exception. Part of that is most people I know tend to just drive places, so I'm not likely to find myself on the train with them. I've flown on the same flights with many people I knew, but it was never a planned experience. I like both methods of getting around. I'd say travel with a companion is generally more fun, but solo travel is far, far more relaxing. Most of the time though, others want to travel on a different schedule than mine, or simply don't have the money/want to spend the money to go places. For me, I start off with a $3,000 travel budget every year, which is my estimated savings by not owning a car and using it to commute daily (as an employee of the local transit agency, I have a free pass to the system, and bike and use the local equivalent of Zipcar as needed). Part of my justification for continuing to be car free is that I enjoy that money much more when I'm using it to take trips, than sit behind the steering wheel of an automobile. In any event, most people I know don't make the same choice in that regard, so I tend to do more traveling than most.
Anyway, back in the days that I worked for an airline (which feels like forever but really was six months ago), I decided to take a solo trip to Russia. I was looking for interesting places to go that would normally be very expensive to get to, in order to take full advantage of my flight benefits. I did the paperwork, obtained a visa, listed myself for standby with Lufthansa and packed my bags. I was flying JetBlue (my employer) to JFK then taking Lufthansa, flying to Frankfurt and on to St Petersburg. I came back by way of Moscow.
I sat down on the JetBlue flight next to a pilot who was commuting to work. "Where you headed?" he asked.
"St Petersburg," I replied.
"Ah, going to relax in the Florida sun?"
"I'm actually headed to Russia."
He took that in, and then asked, "You're headed there by yourself and your not scared?"
I replied I wasn't, and didn't think anything of it.
On the flight to Frankfurt, I sat next to some students on their way to Italy. I mentioned I was headed to Russia, and one remarked, "Wow, I'm surprised your going by yourself. I'm don't think I could ever do that." I also thought nothing, until I boarded my flight to St Petersburg. I was sitting next to a Russian man, and he asked me where I was from. I replied "the US" and he asked if I was traveling alone, to which I nodded. "You must be very brave!" he told me.
You'd better believe that by the time I stepped off that plane, I was pretty worried.
Of course it turned out to be a fantastic trip. I had the opportunity to ride an overnight train between St Petersburg and Moscow, and I was probably the only one who spoke English (and I don't speak a word of Russian!). Also cost me a fraction of what Amtrak costs - I think I made the journey for $47 US (I had my own bed, but not my own compartment and no meals were included).