Yes!

I used to take the Metro to work. Filled with business men in suits, some would get hard while taking a nap, I’m guessing because of the vibration?
Hmmmm this is why we get hard during sleep:
The two leading theories behind why men get erections in the morning both center around hormone levels and increased blood flow: Norepinephrine production dips during sleep. (This is the chemical responsible for keeping testosterone in check) Nitric oxide increases during REM sleep.