I'm not sure it's even that. I frequently catch myself with an expression that looks like I might be dejected, or sad, and the only reason is because it's an ef

I'm not sure it's even that. I frequently catch myself with an expression that looks like I might be dejected, or sad, and the only reason is because it's an effort-neutral facial position: I'm neither smiling nor frowning. Many city commuters may simply be thinking about other things than expressing (or feeling) emotions during their journey.