The cab side windows are only openable on the older CR4000s. The variobahns (which this incident, and the other recorded one from just after the crash, took place in) don't have opening windows, but do have aircon (as do the CR4000s)
You are not quite correct.
The CR4000 (older tram) has windows able to open to left and right of driver. There is no air con in the cab, but it has a retro fitted 'cab cool' which is very noisy and ineffective in hot weather. It is not air con, just a 'cooler' so 25 degrees becomes 22, for instance.
The Stadler, newer tram, has one side window, it is very hard to open. One has to straddle over the controls and use both hands to open it. If you are short in stature, its even harder to open. The tram has air con in the cab and passenger saloon, which when working (most of the time) is very effective. So much so, if the air con does not work in hot weather the tram is often withdrawn as the large windows make the tram unbearably and dangerously hot for passenger as well as the driver.