A starter set engine with a single motor and a couple of freight cars will run just fine on a 40-50 watt transformer. But try to run a Post War GG-1 or F-M with twin A.C.-"PullMor" motors and a few lighted cars, and you're severely under-powered.
Most trains today use LESS power than in the 1970's, because today we're running DC-Can motors and LED lighting. But today we're running more accessories, longer trains, and more powered units than any of us had back in the 1970's, so we need more power.
A KW is a good start. A ZW is even better. On my small layout, I've got a ZW, a KW and an R to handle trains and accessories. The MTH Z-4000 is also a great choice.
Jon