The problem with that circuit is that it's a "Phase Limiter" that controls the voltage by limiting how much of the sine wave gets through (controlled by the switching on/off of the Triac). It's more commonly known as "chopped sine wave". It will work with the old Lionel Universal Motors aka "PullMor", but if you have locomotives with sound boards like QSI/Proto-1, it will drive the boards nuts (I'm not sure of any damage that might happen). Used an identical circuit over 40 years ago to control heat guns. It's better for 120VAC motor control.
The most effective way to control the output voltage and get a pure sine wave is to use an "Autotransformer" on the primary side. An autotransformer is a variable step-down transformer that typically operates from 0-100% of line voltage or 0-140 volts AC. The problem is they're pretty expensive. The Right-of-Way power supply used two of them, plus two 120-to-24VAC step-downs. They were great, heavy, and expensive.
