Why? Stopping the trains wastes energy regardless of whether you dump the extra power into the wheels with friction brakes or into a resistor bank. Dynamic brakes on diesel trains already sink the power into resistors.

Why is moving the place one dumps heat from below ground to above ground not acceptable?

Because you can use it for something else other than heating, like, putting it back into the grid, storing (either battery, flywheel or supercapacitor).