richhotrain
The recurring suggestion in this thread is to drop the A/D track idea. But, I have always understood the A/D track to be appropriate, if not essential, to good yard operation.
The hobby has relentlessly pushed this idea for 30 years, partially in design articles, and then from the misinformation in those articles filtering through discussion forums and elsewhere. Yards small enough for inclusion on most of our layouts, even including club situations, almost never had anything resembling dedicated arrival/departure tracks. None of the prototype yards where I've railfanned in my area have them, including the division points. They're certainly a feature of hump yards, where trains aren't going to move across the hump to do work. How many of us model those? Despite the lack of support from prototype yard functions, the idea has become sufficiently ingrained among some design afficionados that it's taken virtually as a commandment. Unfortunately some of the people engaged in that groupthink have written influential articles.
In model situations, the removal of flexibility from taking tracks out of service from the main body of the yard can make it WORSE. I've seen model yards that got unnecessarily plugged for want of an extra body track at times, but where the owner stubbornly insisted on keeping the A/D tracks free for arrivals and departures only.
An arriving train uses the A/D track to drop off its cars, so that the road loco(s) can then head for servicing. A switcher eventually arrives to remove the cars to the classification yard and the caboose to the caboose storage track.
Once a departing train is assembled on the classification tracks, it is moved by the switcher to the A/D track and then the road loco(s) arrive to remove the assembled freight cars to the mainline.
The operating scenario thus described is common on model railroads, but not really so on the prototype.
The train will arrive on whatever track is designated by the yardmaster (assuming there is one). Road power will be cut off and moved, then the yard crew goes to work breaking down the train to the other tracks, and putting the caboose away.
When a train is ready to depart, it will be assembled wherever is needed. Oftentimes this will consist of the road power and caboose being tied onto a block already existing in the yard somewhere, or they will double two or more blocks before pumping air and performing the required departure inspection. Sometimes the process of combining blocks is done by the yard switcher, other times the road power will do it. The latter may be done by the road crew or yard hostler depending on local union agreements.
Pulling cuts out of the class tracks, and moving them to A/D tracks, can slow operation in a smaller yard. Big terminals have arrangments of runarounds, escape tracks, separate leads, and so on that facilitate these moves.