Thanks alot, that cleared it up tremendously!You are thinking more of walking or driving, maybe, but in the prototype rails and on our hobby layouts, it is merely a siding with a turnout at each end that allows the engine to escape, including via the track on which it first dragged the cars with the engine leading.
Suppose you come to what are called 'facing points', where you have to drop off some cars trailing you. It might be a sawmill, say. It is a small operation, and there is only the one spur into the property, and because you are leading, you must tow all the train into the spur....hopefully nothing too long because you don't want to confound your work in the property, but you also don't want to have to leave a chunk of your consist out on the main blocking traffic.
For our purposes, it is you, the locomotive, and six cars. You leave the main by taking the facing points diverging into the sawmill's spur. If there is just the one in-and-out track leading from the main via that facing point turnout, how will you drop the cars and get back to towing the rest of the train to points beyond? The solution is to have a 'run-around' track. Think of a smallish siding paralleling the track on which you entered the property, but this one has a turnout at each end allowing the locomotive to enter that siding at either end. If you lead cars in, you drop them between the turnouts on the runaround, but on the parallel spur, proceed past the far turnout, back down that run-around, through the first turnout, line the points for the through route once more, and shove the string of cars you just dropped forward toward hoppers or loading ramps.
Note that the need for run-around functions is critical in large classification and switching yards where there are often long strings of cars being coupled to form long mainline trains. Often this can be done by backing out to a ladder lead and taking the next free track over. Sometimes there really are short run-around tracks meant to be left clear for just that purpose.