Last week we looked into the 1:43.5 scale, WELL we now know WHY they switched 1:48 "O" scale to 1:43.5. And surprisingly it's because of HO scale!!
HO scale was the largest model train scale from about 1950 to about 1990. In fact, it owned over 50% of model train sales. And yet 2 rail "O" scale was still a large part of the scale railroad market. And model companies like AHM (Rivarossi) wanted to offer their line in both scales.
NOW, we need to look and how plastic models are made... The injection molds are milled from steel mold blocks. The negative shape of the model is carved into the block by a milling machine with a reduction pantograph. The pantograph had a tip on one end and a small rotating tool at the other that carves into the mold block. As the stylus (tip) moves up the tool carves down. In this way, a prototype model can be placed under the stylus and a negative of the model will be carved into the mold block. However, the prototype need not be the exact same size as the finished model. An oversized prototype model can be used and the pantograph set to a proportion of the reduction. A 2X oversized pattern model can have the pantograph set to 1:2 reduction and the mold will come out the right size.
So if the pattern model is built to 1:29 scale the pantograph can be set to 3:1 and the mold will come out at 1:87 scale and make an HO scale model. Set it to 1.5:1 and the mold will produce a 1:43.5 scale model. And so... 1:43.5 became the standard "O" scale for these models to be offered in both HO and O.
And later, much later, these 1:29 scale prototypes were used to produce 1:29 scale models that run on #1 gauge track. But that's another story...