Trivia: Railroads Used Alpine Lakes To Climb Higher

Here's a bit of trivia for you. In Colorado, two railroads circled a lake to gain altitude for their main line, one narrow gauge, the other standard gauge. Probably the better known among the two is Trout Lake, which was circled by the narrow gauge Rio Grande Southern. What other railroad circled a lake in Colorado to gain altitude?

Comment in with your answer! Or, if you don't know, check back on Thursday to view the answer.◊