For the fun of it, it would be interesting to add a mars light circuit to the Williams Metroliners. At 15" also they work nicely on small layouts.
Here is a video of my Williams set (1 car is missing as I robbed parts as donners for other projects).
I also have a Welz set, but with a Lionel Pullmore motor drive, I don't think it is compatible with modern electronics. I like this set better even if the Williams set is a much better running one.