On New York City subway cars, there is a white mark on each end of each axle. The shape of the marks is as if one divided the diameter of the axle into thirds, drew the perpendicular to the diameter at one of the third marks, then painted from the perpendicular to the near edge of the circle - a sort of smile shape. I am wondering why the marks are there - are they to easily show that the train is moving ? Or do they have to do with balancing the wheels ? Something else ?
I'm having trouble visualising what you are trying to describe, but these marks sound similar to marks that are painted on the wheels of some locomotives and rolling stock, here in the UK. These take the form of three or four radial white lines on the wheel faces, from the edge of the wheel to the centre. As such, they cross the boundary between the tyre and wheel centre, and any radial movement between the two will be immediately apparent by the lateral displacement of the lines. Tyres are shrunk onto the wheel centres so that they are effectively locked together as one. During braking or, additionally, acceleration - in the case of motored axles - the rotational forces between wheel centre and tyre are considerable. Any suspicion of movement between the two needs to be dealt with immediately, and painting lines on the wheels is the easiest and simplest way of detecting it.
I like that other answer BUT it doesn't apply to why the white lines are used on NYC Subway car wheels. You are generally correct in your statement: are they to easily show that the train is moving ? There have been occasions (rare ones) where not all the brakes release completely, as a result the axle would not be moving as freely as the others (or not moving at all) and could result in damage to the entire truck assembly or even a derailment. There also could be a situation where the wheel becomes loose on the axle and spins around by itself. This could be spotted when the train is not moving (but the wheel still is). In either case, should one of these situations be spotted while the train is in operation, it is taken out of service right away.
So here in Minnesota we call them: Ore boats Lakers, if they stay in the lakes Salties, if they travel the oceans
