Dan,
That is a debate that goes back to the 1930's and has been discussed thoroughly. The simple answer I think is that when Lionel offered it's scale Hudson in 1938, 1:48 was much easier to calculate than 1:45. There was a movement among smaller manufacturers to build 17/64" in the 1930's, but as 2 Rail O has always been less popular than the more common 3 Rail, there wasn't enough momentum to make that a US standard. If you look at MTH's European equipment it is scaled at 17/64" or 1:45. The NJT ALP-46 is a 1:45 model as it is based on a European prototype.
I know one of the Lionel experts through my local TCA Division and we have a meeting on Saturday. I'll ask him and see if he knows why Lionel choose 1:48 for their Hudson.
There was some debate on here recently about "O" scale so for the sake of simplicity, it is probably safer to refer to proportion. On30 and On3 are still O scale at 1:48, but run on 30" scale and 36" scale track respectively.
