Those first D&RG 2-8-0s, that went to the EBT also had quite a bit longer wheelbase than the later class 56 and class 60 2-8-0s. However, a better explanation is that the panic of 1873 hit and the D&RG didn't need the bigger locomotive due to a downturn in business. The #13 Mosca was built in 10-1873 and probably had been ordered earlier in the year and probably wasn't needed by October. The Vulcan 0-4-4-0T showed up in June 1873 from England and receive the #13. It was three years later before the D&RG ordered another locomotive and then they were class 40 2-6-0s. The first class 56 and class 60 2-8-0s showed up in 1877 and they weighed 56,000 and 60,000 lbs compared to the 52,000lb for the #13 Mosca. The class 56 & 60 locomotives ran on the 30 lb rail.
Dave