You can decrease it on a service level by adjusting the max service speed in the 'start service' node. I'm not sure if you can increase it above the vehicle design speed, I guess this should be done in an RVD. Vehicles will still obey maximum line speeds, I have no idea how to increase these on existing routes (or if this is even possible)
The start service node? Where's that again? What is an RVD? I think it's not possible to do it on existing routes, but I think I've seen some people do it?
If you create a scenario or a timetable, it's the first node of a new service. RVD is a Rail Vehicle Definition. It contains most of the properties of a rail vehicle and its behaviour.
