I took a look at the Lionel Engineering Standards to see what I could learn about gears. Nothing in this book is dated, but later items appear yo be in chronological order. I believe Lionel discontinued maintaining the Engineering Standards in the mid 1960s. For the 2333 gears, 2333M-14 and 2333M-17 were machined from cold rolled steel. The spur gear on the worm shaft, 2333-46, was machined from brass. An interesting point is that all of these gears have 14.5 pressure angle. All the 2343 gears have a 20 degree pressure angle. This would make the 2333 gears and the 2343 gears not compatible. Lionel appears to have changed from the 14.5 to 20 degree pressure angle on all their gearing in about 1950
The 2343 gears, 2343-124, 2343-111, and 2343-41, are all sintered. I do not have access to the compositions of the materials used in the sintered process, but my guess is that -41 was brass and -111 and -124 are steel. These are all 20 degree pressure angle gear profiles. Gear 2343-196 showed up late in the list. My guess would be late 1950s. None of the other 2343 gears show up again. It is made from Zytel #101 (nylon risen). Somewhere I read that the -196 gear was introduced to reduce the noise made by the horizontal motor F3 units. The sintered gears are not made with the precision of the machined gears and would be noisier.
Based on the sequence of the listings in the Engineering Standard, the first Lionel plastic gear would be the 1122-89, followed by the 2343-196. These were followed quickly by the burro crane, track cleaning car, tie ejector, 68, 264, and lots of the accessories. I do not see where any plastic gears were used in any locomotives other than those listed above.
