Sometimes it happens without me even realizing it, for example while reading a book or just staring out the window. Sometimes I even feel it coming and resist or just let it happen.
The amazing thing is that although I nod off in a kind of semi-hypnotic imaginary dream world, I remain erect and do not fall over. The bumps and jerking motions shift my body slightly to the left and to the right, but I never end up in the lap of the passenger across from me nor on the shoulder of the person next to me.
On occasion I dream of eating and wake up with my mouth open and my jaws biting up and down. Or if it is hot and I'm drinking a refreshing ice-cold substance.
I've trained my mind to sleep but to be alert at the same time. I do not know exactly how this mechanism works, it is like clockwork. Although the train stops along the way, I do not wake up or at least I believe that I am not awake until just the right moment: when the conductor announces Station Gouda! And when this announcement is not made or the public announcement system has short-circuited? I still wake up.