There is a good overall description of bogies and wheels and all things in between at: http://www.railway-technical.com/whlbog.shtml It is well worth the read.
Basically the webpage states:
Not sure if this is the exact definition of a flat tyre but interesting reading none the less.
No. Just no.
Grease pots on curves exist purely to reduce the noise. They serve no other purpose. Steel on steel has the same friction effect as ice, that is 1/5 of the friction that a rubber tyre has on a bitumen surface.
Flat spots, skidded wheels, and related wheel damage are caused because of increased friction from between the rail head and the wheel. Wheels are made of softer steel, and designed to wear away, as opposed to the rail head as wheel are easier and cheaper to replace then rails. When the movement of the wheel is impeded (such as sticking brakes, handbrakes left on, etc etc) the wheel (or "tyre") will rub on the rail head, generating friction and an LARGE amount of heat. Left untreated, you can get flat spots that grow in size proportionally to the length of time that the wheel is allowed to exist in that condition.
The "Tyre" is sweated onto the axle, that is it is heated up, applied to the cold axle and allowed to cool. Forgoing any metal issues (such as pitting, cracking, structural issues) it is impossible for contamination to enter the tyre layer.
Grease traps and lubrication points, when they overly apply their grease only cause wheel skids when the driver applies the brakes and because of the grease layer between the rail head and the wheel skid along, as their is insufficient friction between these surfaces to allow the transfer of braking force to stop the wheel. The action of the grease trap itself does not cause skidded wheels- it is the locking of the wheel to prevent free movement that does.