No, I don't carry a whistle. And I don't carry pepper spray either.
When I speak about solo travel I am often asked whether I bring anything for self-defense. I don't. I wouldn't want to use pepper spray and I likely wouldn't have the whistle handy when I actually needed it.
So what do I do for safety? Well, you'll see that I do lots if you read the Solo Travel Safety section. And, to be specific, instead of a whistle I use my voice.
Engage Your Voice in Solo Travel Safety
I am pretty good at not placing myself in dangerous situations. I stay in public and I choose with whom I speak. (Two of my 5 Principles of Solo Travel Safety.) But there are still occasions when things happen beyond your control.
A few years back I was on an overnight train in Europe in a regular compartment. There were a few people in it. A mix of men and women of different ages so I did what made sense on an overnight train. I went to sleep. However, I awoke in the middle of the night to discover that all but one man had disembarked and that he was pawing my body.
I yelled. And I went into the corridor and yelled. I got the conductor and even with the slight language problem, the tone and level of my voice told him what was going on. He moved me to another compartment where I was safe.
I may not have had a whistle, but I did have my voice. And I wasn't afraid to use it. If I'm being bothered by someone, I make it known. And, given that my policy is to stay in a public, there are always people around to come to my defense.
Find Your Voice
Some people, women especially, have naturally quiet voices. And they may be afraid to make a fuss. You need to get over both. Before leaving on your trip find your voice. Go into a basement or some place where you can yell and go for it. Practice using your voice so that it is ready if you need it.