Never gave this much thought even though I'm a daily Metrolink commuter. Interesting articel...
http://www.citylab.com/navigator/2015/10/ask-citylab-why-wont-subway-doors-reopen-when-i-squeeze-through/410365/
http://www.citylab.com/navigator/2015/10/ask-citylab-why-wont-subway-doors-reopen-when-i-squeeze-through/410365/