I have now invented the "trundleduct". I will stick some pics on my website when I have a minute.
I am building a model railway in my garden room, which is 27' x 17' (8.2m x 5.2m or four by two and a half centifurlongs). It goes round the room twice, crossing over itself, but it has to cross in front of a double door which is how you get in the room.
Across this door will be a viaduct, 400mm high (100ft scale, roughly) and about 5ft long, which is on a frame mounted on wheels which fold up when you move a lever, and swings round on a pivot.
So: viaduct which trundles out of the way, or trundleduct.
I'm thinking of patenting it...
Guy