When using the 766.2 cabcar propelled by a BR143 you can reach (and exceed) 130Km/h despite the 143 Max speed being 120km/h. This isnt just when maxing out on a downhill section but even up hill.
Max speed isn't usually defined by how fast can the X vehicle go if you stay flat out for long enough (apart from top end performance cars). If the BR143 has a max speed of 120km/h is because of other structural reasons, probably something that has to do with the design of the wheels, bogies, suspension, panthographs, etc... coaches have a max speed too, but theoretically you could couple a 1930s coach to a Taurus and go all the way to 230km/h, but it wouldn't end well. It's your duty as a train driver to know the max speed of your train and make sure to adhere to it, even if physics won't stop you until it's too late.
The 143 won't go over 120 easily at all and tops, whereas when propelling a cab car just doesn't act anything like the same physics wise. I mean if that's prototypical then fairdoes but I find it very bizarre of that's the case.
When you operate the BR143 directly you use AFB, so you select the speed and it will reach it and hold it to the best of its very limited abilities, even if it overshoots the set speed is going to be on it's way to idling, and will follow with some dynamic braking. If you control the BR143 from the cab car you are directly requesting a certain amount of kN of force from the motors, and the BR143 will do its best to achieve that force. In that case there is no check or limitation for speed and is completely up to you. Keep in mind in BRD lot of the services with the BR143 are only formed of a coach and a cab car, so the train will feel much faster and responsive compared to the classic RSN consist (with the exception of the mighty Iserlohner!)
It's around 15-20 years ago, but I once rode the RE50 from Dresden to Leipzig, standing right behind the cab in a 762 Dosto cab car, 143 at the end. Driver and guard were talking in the cab, thus not really looking at the speedometer. The train accelerated to more than 120 quite a few times, once even got braked down when the PZB came into effect at 145 km/h, since the cab car had a top speed of 140. I double checked in Leipzig, but it really was a 143 at the end. It is surely not allowed to, but the 143 is capable to drive more than 120. In fact. German trains are required to run 10% more than the desired top speed in test runs.
