Welcome to the forum!
You know, there are folks out there that are way smarter then I am when it comes to this stuff. When I get in a situation to where I have a question and I have no idea what the answer is, I'll often hit the search function at the bottom of this page. If you plug in the words "Drop Leaf" or "Lift Out" or "Swing Gate" you'll have lots of information (I checked!).
Please don't hear this as a blow-off. It's like, either I can tell ya, or you can hear it right from someone else......your choice!
Welcome aboard!
Phil's idea is a good first step. This topic comes up from time to time, but we always are on the lookout for new ideas and variations, too.
One question I have from the original post - do you plan to make the door itself part of the layout, by actually attaching the track to it? That would be a new twist. Most swing-gate setups I've seen are actually only a half-door, from the track down. Either way, to make this work you need tight hinges so that the door's motion is exactly the same every time.
It takes an iron man to play with a toy iron horse.
There are three practical ways of doing it:
1. Mount the railroad on the door using shelf brackets. I would keep the railroad as narrow as necessary to minimize the weight hanging on the door. Door construction should be toward the heavy side with a solid door best in my opinion. You need to consider if the door opens in or out of the railroad room. It will be virtually impossible if the door opens into the railroad because there will be nowhere for the railroad to swing without hitting the adjoining fixed benchwork.
2. A liftout section. easily done but again keep it small. Easy to drop and possibly damage. A couple of vee shaped blocks under the railroad will allow it to self center itself if the fixed wood pieces are adjusted properly. Use screws on the receiving blocks so they can be adjusted if needed and connect the wires to a plug so they can be connected easily. With a few more connectors in the plug you could also eliminate the power to the fixed rails for a couple of feet either side so the trains aren't susceptable to visiting the great chasm.
3. Same idea as #2 but fix one end with a hinge and drop the other end into a vee shaped block to allign it. A framing nail can be used as a pin to lock the bridge in both the up and down position to prevent it falling.
4. Anything but a duckunder. The older you get the harder they become to get under. I am to the point that I am building a railroad that will have aisles four feet wide or wider and no area will be inaccesible unless I am standing upright. I will have one fixed bridge with a hinge and a contained space for the yardmaster to operate from. most of the railroad has eight feet of space available between sections. All this only possible because of being an empty nester with a full basement at my disposal.