Lionel machined rollers out of steel for many years. These are the rollers with the stub axles on each end of the rollers. As volumes went up and there was pressure to cut cost, Lionel went to sintered steel rollers. This was about 1949 or 1950. These are the rollers with the roles all the way through them. When Super O track was introduced, there were complaints about the narrow center rail cutting grooves in the rollers. Lionel responded by changing the sintered steel roller material to stainless steel. But there were millions of the sintered steel rollers in service and in the spare parts network. Even today it is not uncommon to find parts dealers still selling NOS sintered steel rollers. Rollers are cheap and easy to change, so I have never seen where roller wear should influence any decisions about Super O track.
An interesting design question with rollers I have never gotten answered is there are different part numbers for a couple of roller designs that are dimensionally identical, but made of different material. The rollers used on two motor locos were a different composition than rollers used on one motor locos. In the engineering standards the different materials are only identified by the Lionel composition identification number, so I have no idea what the difference in the material is. I am sure once they left the factory as spare parts the dimensionally identical rollers would have gotten mixed regardless of composition.
