# Why moving train's windows slam shut when another train passes slowly in the same direction?

My question is related to this one but distinct as this case is trains travelling in the same direction.

We're familiar with this scenario. You're sitting in train A and another passes at high speed and your windows slam shut:

===[ train A ]=== stationary
===[ train B ]=== -> 60km/h


Often when tran B passes a bit slower the windows do not slam, e.g:

===[ train A ]=== stationary
===[ train B ]=== -> 40km/h


I'm interested in the nuanced scenario which I observer regularly, when you're travelling at medium speed in train A and train B passes relatively slowly, yet all the windows slam shut:

===[ train A ]=== -> 40km/h
===[ train B ]=== -> 45km/h


What is the explanation for this behaviour? These are the sort of windows I'm talking about - opening inwards with a reasonably strong spring that tries to keep the window either open or closed.

The windows slam shut due to Bernoulli’s principle, which states that an increase in the speed of a fluid results in a decrease in pressure. When a train is moving, air outside the train is moving faster than air inside the train, thus pressure in the train is higher. When the pressure difference is high enough, the windows slam shut. In the case of one train passing another at speeds of around 40km/h, even though the relative velocity is lower than when a train traveling at 60km/h passes a stationary train, the winds produced by each train can add to each other and boost its effects, just like how in wave interference two waves superimposed can have their effects added together. The final wind speed between the two trains is enough to cause the windows to slam shut.

• Great answer, thanks Eiekama. I'm now one step closer to being an insufferable know-it-all. – geotheory Aug 16 '19 at 9:06