I have a 2000 copy of the Trains Unlimited, Tours brochure. It says "The 3-foot gauge National Railways of Guatemala is back open and being operated and built by Railroad Development Corporation of the USA under the new name of 'Compania Desarrolladora Ferroviaria'." There are photos of the K-36 going over bridges as well as a photo of the roundhouse and US export diesels within. The Mikado doesn't look itself with that tiny headlight on top of the boiler...
Here is a link to the Trains Unlimited website. They have a photo gallery and will send a free brochure. I have my brochure simply for the photos, but some of those trips look pretty neat.