6 Wheel tenders were incredibly popular until 1880 or so on both standard and narrow gauge railroads in the US, just not on the Rio Grande. Some say it was believed they offered a softer ride at higher speeds. Looking outside of the Rio Grande sphere (or even look at the early Grande locomotives in Utah) you will see the 6 wheel tenders either in a 4-2 (1 bogie and pedestal) arrangements such as shown here, or 2-2-2 (3 individual pedestals) such as those on the Colorado Central Porters were all over the place.