The Variac is a good approach for a continuous application. The specific situation I am trying to control will be at a county fairgrounds facility, and will have a total of 12 single tracks with trains. The transformers planning on using are small inexpensive ones, and conventional engines only. And, only be in place for 2 1/2 weeks. So cost is a factor. I think the pot on the output would be best if I get the correct rating.