Why don't more designs have an Interchange track?
What is an Interchange Track (in the prototype)... in the modelling world....
What are the benefits of having one on my layout?
Staging and Interchanges....
Operations and Interchanges....
Many (all?) of us have started with a simple loop of track. This is what comes in all those starter sets, and this is what many of use have going around a christmas tree. For many, many people, that loop of track is enough. Building a small town or diorama and having the train run around the outside 'scratches the itch' for them. But for many others, we need something else. We want that "model layout". We want to model a railroad (or a small section of railroad at least). There are three elements necessary to if you want to model a real-world railroad, because these three things are on every railroad:
An Industry (usually on a siding or spur): The purpose of a train, and the railroad, is to move goods and/or people from one place to another. So you need to have that place that you are moving to/from. It can be on a spur (a single turnout, dead end length of track) or a siding (a length of track with a turnout on each side).
A Run-around / Passing siding: As far as track goes, this are (usually) the same thing. A siding is a length of track with a turnout on each end. A passing siding allows one train it sit in the siding, while another train passes it. A run-around is a maneuver where the locomotive disconnects from one side, 'runs around' the rest of the train, and connects back up on the other side. This is more often than not done at a siding, but technically you can use the whole loop a 'run-around'.
An Interchange: This is where one railroad connects to another railroad. Where one railroad will drop off cars for the other railroad to pickup. Almost everyone will add the first two things to the loop, but the interchange track seems to always be forgotten. And in my humble opinion, it is the most import element of a model railroad. There are only a VERY few railroads that are completely closed to the outside world. And even if your industries are all along the same RR line (for example, everything is served by Norfolk Southern), you can't possibly model all of the NS line - you have to have some "off layout" track.
Trains very rarely run around in circles... they travel from point to point. And along that travel, they are picking up and delivering goods to the industries that they serve. But very few railroads serve every need of the industries along that railroad. For example, a railroad might serve a steel mill, but it probably does not also serve the mine that brings in the ore. Or the railroad could deliver goods to a grocery warehouse, but very unlikely that the railroad serves all the different companies that make/grow the food to be delivered to the warehouse. And since railroads don't tend to run on eachothers tracks, this is where an Interchange spot is needed.
The railroad that serves the mine will haul the ore to an interchange location (and usually pick up the empty ore cars left there from the last run). Then the railroad the serves the steel mill will pick bring the empty cars from the last run to leave and grab the full cars to deliver and take to the mill. The same is going to hold true for almost every industy on a railroad. The interchange track is what connects the industries along one railroad to all the industries on another railroad.
So you have your loop of track, and then you add your industry on a spur. Your industry could be anything: a steel mill, a goods shed (freight warehouse), a factory that makes widgets, a passenger station, etc.