Caddy58 has created this thread about his Tidewater Flats layout. It is very inspirational just as a layout construction, but it also has a great example of a swing out (down) section of the layout used for access. This device precludes the use of a duck under for your layout. In particular, this gate is large and very sturdy with adjustment screws. This is a thread well worthwhile for those considering such a device or wishing to avoid a duck under. Additional search terms: yard duckunder staging backdrop bridge car load drop-leaf drop lift
Here is another approach to a swing gate. It is by Ryan Boudreaux and is probably the most complete description of construction that I have seen. More threads: Lift out or swing gate need help. Lift out / swing gate alignment. Swing Level Sections