Sounds like they'd be new CAT engines. It seems possible that they could evade FRA Tier 4 compliance requirements since they aren't used inter-state commerce.
Here's the relevant section of that article:
"DURANGO, Colo. — The Durango & Silverton Narrow Gauge Railroad is purchasing two, new three-foot gauge diesel locomotives for $3.2 million from Motive Power & Equipment Solutions in South Carolina. The MP2000NG locomotives will feature 2,000 horsepower CAT engines with General Electric 764 traction motors capable of pulling a loaded, eight-car train along the 45-mile route between Durango and Silverton. Railroad officials expect both locomotives to arrive in Colorado in Spring 2019." - From Trains Magazine web site July 31 article.
Will they be shipped via truck the entire way across country? That would avoid the extra time and expense of loading/off-loading onto standard gauge flatcar and likely be quicker. Fun stuff to ponder...