I believe the quality was there in the post war production. Even in the trains produced in the last years of the original Lionel Corporation, up to 1969. I'll go even further and say that the Fundimensions or MPC era did their best to produce a quality product.
That said, quality can be defined differently in each of those three eras.
I would venture to say that the highest quality trains were made between 1945 to 1959. Robust motors, heavy materials, more detail, magne-traction, etc.
In the period starting in 1959, the trains became more toy like. Cheaper materials started to be used. Basically the designs were changed to make manufacturing simpler, which in turn made the trains more toylike.
After the Lionel Corporation went belly up, the new owners continued with the less expensive line of trains. Perhaps feeling their way in order to see if Lionel trains would even continue to exist. Fortunately they must have been making a profit and started producing a more desirable line of products as the '70s wore on.
Toward the end of the '80s, more bells and whistles, so to speak, began to appear in Lionel trains. I recall some difficulties starting to show up with these more technically advanced trains in the early '90s.
The products produced today have far more electronics than a post war loco could have ever dreamed possible. If you've followed some of the topics on these forums, you may have noticed many complaints about some of these high tech trains. That's not to say the trains are not superb models. It's just that there's more to go wrong than in a 1950s locomotive.
The test will be to see where todays trains are 60 or 70 years from now.