Pratchett announced it by holding up a card saying RAISING and then another saying STEAM. The previously-announced Moist book was RAISING TAXES, so this was clearly a nod towards that.
Pratchett's been saying that RAISING TAXES wasn't coming together very well, so my guess is that he's diverted Moist from the tax office to being in charge of The Undertaking (Ankh-Morpork's impending underground railway network, its version of the London Underground), allowing him to kill two birds with one stone by also cracking on with the Undertaking novel he's been hinting at for years now.