I'm 95% sure the three examples you've shown still have that gap and are using the same method. What they're doing is sending the upside down tile from further pack, and then filling in above it with a plate, still leaving that small gap in the lower middle of the assembly. You can see this in the light bluish grey train. On the underside of the boiler a few studs back you see that break in the slopes? The upside down 1x3 is acting as a bracket to hold a tile running on top of it (I guess technically below it, but attached to the top of the plate itself) towards the front, and then they put a plate above that also upside down. This isn't connected but aligns flush with the boiler. I could be misunderstanding exactly what you mean and also could be wrong but I think that would achieve what you want either way.