Once again, I get to comment on where hobbies collide.
If you happen to game in 20mm (i.e. 1:76/4mm to the foot/00) and the appropriate period, this just turned up on my railway modelling FB feed....
No more details on the Oxford Rail site, but it would appear from comparing photos to be this beastie, the Ordnance BL 18 inch Mk I howitzer on truck, railway (Boche, destruction for the use of :D) along with a Railways Operating Division Dean Goods.
Not entirely sure I can justify one, but I'm sure someone out there who plays CoC in 20mm could come up with a scenario... I'd also note that I don't believe the price, since a Dean Goods from them on its own is way more than that!