There's an axiom in layout design that the yard lead should be as long as the longest track, but I just found second reference to the fact that's not the way they did in the prototype. The first time I saw the reference to running cuts, rather than complete trains was because it was kinder on the switchman's feet (and faster, given how far he had to walk). I was never sure how one worked, since I assumed he'd hitch a ride on the last car in the cut, but when a credible (published) source says it's so, I kinda have to believe. But today, I was reading a Kalmbach article on LDE's and the author noted that, when he was looking at the Nickle Plate's eastbound yard in Frankfurt, IN, he couldn't find a track long enough to be the lead so he asked someone who'd worked the yard BR (before radios) and was told that, at least back then and at least in that yard, they'd never pull more than 15 or so cars at a time, because with more cars, the engineer couldn't see the hand signals - or even the switchman. That one makes complete sense. Any of you who played with 1:1 scale trains able to verify or add anything? And how about today, now that there are radios? And a side question: When the arrival yard, classification yard and departure yard are arranged sequentially (as in North Platte) rather than in parallel is there really a need for a lead? Selkirk seems to have a uni-directional hump so needs the leads on both ends to get westbound arrivals to the arrivals yard for classification and seems to have a pair of leads at the east end to get the cars from the classification yard to the departures yard - but the departure yard is on the north side of the hump, not in series. At least that's how I read the view from "on high" (Google maps). It's mostly about curiosity.
I agree that for flat switching cuts of cars are more manageable and one can realistically plan a yard to fit a space with this requirement. A tail track (SP for yard lead) as long as the longest track doubles the length of your yard causing many to go without altogether. If your operators block the cars well then cuts work out quite well. Most model yards have the arrival yard beside the classification tracks rather than inline with a hump, but if you do have the space the arrival tracks can serve as your lead nicely. I like to have a long throat in my yard that allows the switcher to run independent of the mainline and A/D tracks for the most part so he can keep working regardless what is happening on those tracks. I live, grew up, and now work near the UP (old SP West Colton) yard and everything arrives on the west side and departs out the back of the bowl (hump) eastbound. The westbound trains take a balloon track to return to the intended direction. There is a double mainline headed east for departing trains and I believe one track is considered a lead (902), but is mostly used to move power to the correct track. What are you attempting to accomplish?
John, First, thanks for the information. As for what I was aiming for, truthfully, it was for knowledge, not layout. I have an urban switching layout with a small yard that receives and stores cars for/from local industries using one or two locally based units (second is for the day my grandchildren are older and one says "Grandpa, let's play with your trains." As a "lone wolf" operator, anything more is fun thinking about, but not the least bit practical for me to implement.
Southern pacific East Yard in San Antonio has a west end yard lead that makes a 180 degree turn and runs parallel to the classification tracks. It is a tight, tight turn but I have seen tunnel motors switching on that curve. The yard, including both yard throats and not the east end yard lead, is about two miles long. The TOFC tracks are inside the yard lead. I have never heard of another yard like this.
If you look at Mockingbird Yard in Dallas (just south of I-30 and west of Mockingbird Lane, you will find the east lead to be about 2/3 the length of the longest track. This is due to space more than anything, but it was built by the SP using their practices. We usually only switched about 30 cars at a time simply to make things easier on the switch power and ensure no "pokes" past the signal were needed. The longest track is about 50 cars long. I worked it as a Conductor trainee and then as a Clerk for a few years in the recent past, so I can't speak for how SP or UP did it, only for DGNO.
Cars get switched in cuts, yes. Many real yards have leads long enough to accommodate a whole train so they can move a train from one track to another without tying up the main line (or through tracks). Some smaller yards use the main as the lead. Trains have to be given authority to use the main from the Yard Master. Dispatchers to not control the Main within yard limits, the Yard Master does. There are a lot of variables and you almost need to look at each yard and weigh it on it's own.
