Hi! I am interested in this question as well, so I come up with an idea and used LDD to build a prototype shown below:
It is just a prototype, all the bricks can be altered, it just interprets my idea. I just utilize the friction between the rubber wheel and the tube piece to slow down the rotating axle. And the switch part in red should connect to another small motor to achieve the remote control. The power-output-axle should connect to the axle that the switch is on (whatever how you connect them). When you use this system to stop your car, you should stop the XL motor first and turn the switch, in order to let the deceleration part work. You can see that the wheel deceleration part is split, rather than directly install onto the axle connecting to the motor. I think if I connect them directly, the axle will still stop immediately when the motor is stopped, the brake system won't work. I think it makes sense. According to this prototype, I think its volume may not allow fitting into an 8-studs train.