MTH has just announced (on 5/9/18) a new production run of Big Boys that includes additional new road numbers (4014-Coal, 4006, 4017, 4018, 4023 --- All of these are Coal engines) (Catalog also includes 4014-oil from prior catalog)
MTH new release: https://mthtrains.com/news/613
Download PDF: MTH-OneGauge-Catalog-2018(BigBoy).pdf
Per this announcement new engines will start arriving this July 2018. Depending on quantities that arrive it could be delivered in phases at later dates as the 2015 Big Boy release. Important to remember the 2015 Big Boys sold out in pre-order so if you want an engine you need to get your orders in ASAP to reserve yours today!
I have them listed on my website: http://www.rayman4449.com/MTH/MTH_OneGauge.htm