Trains generally cruise for hours when the motors are not producing peak torque at near 0 rpm. They are also traction limited which means they may not be using peak torque to start. A mechanical transmission adds more weight and complexity for very little efficiency gain.
Cars and formula e are driven in a variety of conditions. They have to navigate the parking lots in los Angeles, slow down for curves then speed up again, cruise across the desert. In many situations, the electric motor in a car would not be operating at its efficient range. This is where a efficient mechanical transmission would help