We don't seem to have proper filler putty in the house at the moment, closest thing I could find was a tub of spackle in the garage. And there might be a tub of a different wall patch compound.

Would either of those be at all workable as filler for a plastic locomotive body shell?

I reckon that you might have some bonding issues between the plastic and the filler. Also the finished texture might be coarser than the flat plastic and could look odd when painted. 
Having said that just give it a go on a bit of scrap and see what happens.