Your right I wasn't looking at the figure properly. It's a pin and roller between two surfaces so yes depending on the situation the reaction force could be either.
Check out this site for learning about the Psychrometric Chart. Yes an increase in pressure can cause water vapor to condense.
Edit: Here's the link http://www.powerknot.com/how-to-read-and-use-a-psychrometric-chart.html
I failed at embedding it.
From what I read it looks like figure 4 is describing a roller or a pin. The pin would be the one with the downward force. The figure isn't that clear but I think that's what it's showing.
Your equation is right, and so is your answer if you assume that the negative acceleration continues for the entire 40 seconds. At 20 seconds the train comes to a stop and then begins to move backwards. At 40 seconds the train is back were you began with a velocity of -20 m/s. They way the...