Narrow Gauge lines.
I had no particular bias in favor of narrow gauge lines, but it seemed that much of the steam that did survive was narrow gauge. West Side Lumber, the Southern Pacific narrow gauge line, and the Denver and Rio Grande used narrow gauge steam in regular service well after most railroads were dieselized. And some interesting tourist railroads like the East Broad Top, White Pass, Cumbres & Toltec, and Durango and Silverton are narrow gauge.
