Hi,
You may need to control the current flowing through a motor.
Simply you can check using a shunt register and comparator with driving transistors.
The voltage out from the series shunt registor will be compared with a reference voltage you set for the speed, and the comparator signal will control the driving transistors.
Once the speed is reduced at the corner, the current flow through the motor will also be decreased.
This will lower the voltage drop across the series shunt and will trigger the comparator to boost up the current into the motor.
since it takes more energy to make the turn, you will probably need to implement some feedback between the angle of the train wheels and your controller.