Not soffit board, but I've used 3 or 5 mm plasticard on Hardyard for the bridge and the refuelling area.
Some was from old shop displays and had a vinyl photgraphic display. Some came from a Laura Ashley shop and some from Class tractors, and the plain white stuff from a mate at the European Railay Group who gets large offcuts for free.
Cut a score line then snap for straight edges. It's lightweight and strong and was used to re-inforce the hardboard.
I've not used it outdoors as yet though as I have only a few (Jackson's Minatures) buildings on my railway....