Couple nomenclature corrections for USA types;
Siding has switches at both ends, Brits call it a "passing loop". Sidings can be used for two trains to pass each other, or for an engine to run around to the other end of the train. A siding in a yard designed specifically for runaround generally ends in a stub called an "escape track", usually at least as long as two locomotives.
A SPUR is a dead end track, usually used to service industries. A "trailing point spur" or "facing point spur" depends on which direction you're approaching from. Trailing point is simple, go past the switch, throw it, and back in. For a facing point spur standard procedure is to run around the car or cars to couple onto the front of the loco, then push them into the spur. Or, if you're picking up, run in nose first and pull the car out, then pull or push to the nearest runaround siding to uncouple and move around so the car is behind the loco(s).
Preperation, if the siding is a long distance away, the "peddler freight" (way freight) crew will usually go to the siding first and move whatever cars are to be set out to the front of the loco, then push them all the way to the spur. Depending on the railroad and the state of the track there are usually speed restrictions when pushing cars about the same as the ones for backing movements.
There are other ways, such as the flying switch and the Dutch drop, but they're illegal on most railroads today because they're pretty hairy.
www.youtube.com/watch?v=IZN-mwHWxiY
Other thing you can do with Trainz, dunno how common it is on the prototype, is to uncouple one of the locos if you have a two MU set - drive the locos separately and leave one on the mainline, use the other to pull past it into the spur, then change back to the first to pull the cars out and let the other loco run past. Probably takes more time, but if the runaround siding is a long ways off it might take less time than running for miles at restricted speed while pushing cars.
