The Denver & Rio Grande owned over 2,000 narrow gauge flat cars all together. This fleet was very diverse, no less than 28 different classes of flat cars have been built. During the 19th century, flat cars were also frequently converted to gondolas, and vice versa, according to demand. The first Rio Grande flat cars were 90 small 4-wheel cars with a capacity of 5 tons, built in 1871 according to Billmeyer & Small plans, they have been used mostly for the construction of the railroad. All the other classes of Rio Grande flat cars were equipped with trucks (except for one car), 17 classes of various length and capacities have been built between 1871 and 1882, some classes being originally gondolas. Most of these cars were retired before the end of the 19th century, the last cars survived until the thirties in the MOW fleet. In 1887, the D&RG received a new class of a hundred flat cars with a capacity of 20 tons and 30ft long, numbered from 6000 to 6099. During the first quarter of the 20th century, several dozens of theses cars were transfered to work service and 4 of them are now preserved on the Cumbres & Toltec Scenic Railroad: #06008 the boom car for pile driver OB , rail and tie car #06051 used during wreck clean-ups, #06063 the boom car for derrick OP and wheel and tie car #06092 for derailment recovery. The cars of this class still in revenue service were rebuilt in 1924 and the last of those not already retired were sold to the US Navy in 1942.
In 1918, the Denver & Rio Grande built 20 34ft long flat cars, still with wooden frames, numbered from 6000 to 6009 (20-ton capacity) and 6010 to 6019 (25-ton capacity). They have been rebuilt in 1937 and their wood frame strengthen with iron. Three of these cars are preserved in working order on the Cumbres & Toltec Scenic Railroad, #6200 and 6214 used on work trains, and #6205 converted to a rider gondola, #6209 is conserved at the Colorado Railroad Museum and #6201, 6204 and 6217 on the Durango & Silverton Narrow Gauge Railroad. The D&RGW shops have built 15 other flat cars, 40ft long and with a capacity of 30 tons in 1926 (#6300 to 6314), being entirely of wooden construction these cars were a curiosity at a time when all the new car built were already steel cars. #6314 is preserved today on the Cumbres & Toltec Scenic Railroad. Between 1940 and 1944, the Denver & Rio Grande Western shops converted 45 42ft long steel frame standard gauge fishbelly gondolas, into narrow gauge flat cars with a capacity of 40 tons and numbered between 6500 and 6544. These cars equipped with Bettendorf trucks remained in service until the end of freight operations on the D&RGW narrow gauge in 1968. Fourteen of these flat cars are now on the Durango & Silverton Narrow Gauge Railroad and fourteen other, converted to passenger cars, are used on the Cumbres & Toltec Scenic Railroad.