The reason for controlling the clearance in the thrust bearings is to keep the commutator from hitting the under side of the brush holder and to avoid impact loading the thrust bearings. And on early motors, to keep the balls from falling out.
There is no change in the strength of steel due to heat until above about 1100 degrees F. Unlikely the brush springs run that hot.
Lionel used three different brushes in the post war era. They had carbon brushes, which are black and fairly light; they had copper graphite brush, which is black with a copper cast to them and are heavier; and they used a copper coated brush, which are a copper color, but the copper wears off the contact surface, which is then black. New brushes are only available in carbon. NOS of the other two types can be found, but are getting rare. Brushes need to have significant resistance to control the current flow when the brush shorts across the commutator slots.
