As was alluded to in another question – there’s very little to be learned from building a “proven” design. Moreover, most of the other classes suggested either still exist, or are similar enough to other extant locomotives that their configuration is already represented in the heritage fleet. There’s no point in building another Berk or 4-8-4 when there are so many running or restorable examples already out there. There are 4-6-4 and 2-10-4 projects ongoing that we don’t want to be in direct competition with for resources or public attention.