Back in the Pre-Z4000 days of overheating ZWs when trying to run show layouts, the $$$ ROW transformer was a commercial option. A lot of folks come up with cheaper options. There was a lot of discussion on boards (remember Compuserve?) about using 120 VAC Triacs and an 18v step-down isolation transformer. There were a number of limitations to the solution including a requirement to have a load resistor (lightbulb) on the output side (can't remember the technical reasons). So I decided to build power supplies using Hammond 18V stepdown transformers with a variable voltage autotransformer (AKA Variac) on the input side - the same circuitry as the ROWs. A successful, robust solution at half the cost of the ROW capable of running 15 amp loads without breaking a sweat. Some assembly required but isn't that why we enjoy the hobby? I now use the autotransformer to trim the output voltage to 18v and TPC400s to control track voltage. An evolving challenge has been circuit protection and considering the current cost of instant trip magnetic or electronic overcurrent breakers, I don't recommend a home-brew solution these days. Off-the-shelf Z4000, PowerHouse and PowerMaster gear is good value with superior circuit protection.
The autotransformer models I purchased 25 years ago are still available at about the same price (~ $75) so that is an option. A search of the net will turn up lots of links to Triac and motor controller solutions. They work, but YMMV.
The best solution for defective post-war transformers is retirement. There's a current thread on a Type R and its internal wiring - I use one on my test bed - but even that needs good output protection added on and after reading of some of the Type R issues, I'll replace it soon with new gear. The days of thermal breakers with seconds before trip time is OK are long gone.
