I think the answer lies in considering overall plant capacity. If you just look at a single train that needs to transfer between two arbitrary tracks, there are indeed more turnouts than necessary. However, if you consider that a busy metropolitan area will be moving many trains in and out simultaneously (several commuter trains, an Amtrak or two, for example), then the plant makes more sense. You need some schedule independence as well -- some trains will be a little earlier or later than expected. That can put more demand on the number of non-intersecting routes needed.
dbduckan example of double slips
.
I have seen that picture many times before, and never understood the track layout. As I trace possible train paths, there seem to be a lot of unneeded double slip switches. I know the railroad would not have buit them if they were not necessary, but I just don't get it.
Maybe if I saw the track plan of the entire facillity I could make it make sense. Maybe not.
I did not understand the "High-5" instersection in Dallas until I drove few it a few times.
-Kevin
Living the dream.
I used a 3-way at my Union Station .
GARRY
HEARTLAND DIVISION, CB&Q RR
EVERYWHERE LOST; WE HUSTLE OUR CABOOSE FOR YOU
BigDaddy don't know who made 3 way turnouts 30 years ago
Russell
I thank all of you for your assistance.
I just wanted ideas of where one might use such complex devices.
I recall a couple of articles in the 70's and 80's where someone whould comment on a piece of trackage indicating a "real" railroan would never do that. Well, within a few days someone would post a picture of the exact, or allmost exact "real world" example.
My 40 years as an electrical engineer, supervisor and manager of the distribution (getting power form a substation to the customer) portion of an electric utility proved that companies would do what was necessary to get the job done. Sometimes, the planned solution might seem expensive. Most usually the designer came up with the most optorune fix.
an example of double slips
Trying to replicate that on a layout would put me in the poorhouse...lol
As cx500 mentioned, double slips are common in the yard throats for passenger terminals. To see this in action, try maps.google.com and search for "chicago amtrak yard" and zoom in and change to satellite view. There you will see several tracks leading to the south platforms of Chicago Union Station. There are many double slips there providing a space-efficient structure for having trains easily transition between platform tracks and approach tracks. (Undoubtedly this same structure occurs in many if not all major cities, but I am familiar with the Chicago area.)
I also appreciate the comment from rrebell. Be sure that you are designing what you want first and then apply the supplies you have where appropriate. You might be disappointed if you go out of your way to build something with all nails just because you have a hammer.
You are going about this wrong, you are designing with what track you have instead of doing a track design and making things better with special track pieces.
The earlier answers are all good. Double slips are primarily used at major passenger terminals with multiple tracks leading into multiple platform tracks. They are a very efficient use of space, but the penalty is initial cost and ongoing maintenance, particularly of the switch points and frogs. They would quickly become a nightmare if subjected to heavy freight traffic.
Three-way turnouts would generally be restricted to yard trackage, especially classic industrial areas where every small warehouse had a loading dock. Ferry slips were another place where they could sometimes be found. If possible the railroad would use two standard turnouts to avoid the cost of the extra frog in a three-way, and on a main track they could usually find enough room to do just that.
In the model context, of course, space is always at a premium and maintenance costs are not a significant factor when the trains are not 6,000 tons of freight. We compress reality in many ways already. Using double slips and three-way turnouts to allow the desired operation is just another compromise that is at the discretion of the modeler.
Double slips were used where space was congested and at a premium. They were expensive and maintenence intensive, so prototype RR would avoid them when possible
George In Midcoast Maine, 'bout halfway up the Rockland branch
BigDaddyI don't know who made 3 way turnouts 30 years ago (read that as a comment on my ignorance and not on manufacturing techniques in the past)
I had some brass Shinoharas that looked identical to the nickle-silver ones made today 50 years ago.
I have no problem with my 3-way turnouts shorting. I use the shorter Peco electrofrogs in a yard, and I've got one longer Walthers/Shinohara for a staging yard throat. I have no problems with any of them. I power the frogs with a Tortoise machine on the W/S turnout.
I've never had a double-switch, but I'd use it for a passenger terminal if I did. I just thing they're neat.
It takes an iron man to play with a toy iron horse.
I don't know who made 3 way turnouts 30 years ago (read that as a comment on my ignorance and not on manufacturing techniques in the past) However the recent 3 ways have problems shorting at the middle branch. There are several threads addressing this and where to put the nail polish to prevent it.
Not the question you asked, but if you are going to use them, you might want to research it.
Henry
COB Potomac & Northern
Shenandoah Valley
Not strictly so (by any means, since railroads were ingenious at devising solutions to rail operations problems, and believe me, they fixed 'em one way or another), but you'd often see double slips in terminal yards, the large passenger variety where trains backed in, or backed out if there were no practical turning methods. The slips allow trains to...well, slip...from one butt-end onto another on their way outta Dodge. This allowed them to get to a final turnout near a throat, or to the throat directly, and then out of the Greater Yard and onto the main for high speed delivery of goods and passengers.
But, you'd find them where necessary, really, and what greater necessity would you or I have except in a smallish yard on your average layout?
Think of the trunk and branches of a tree. The trunk has several main, lower down, large branches which could be throats to a number of facilities, not least of which would be that large terminal. They all have to gather their small branches, these 'throats' and direct them to the trunk, which would pass for the main line, with the roots below ground mirroring what appears higher on the trunk. Double and single slips, and some shallow diamonds, allow trains attempting to escape out to the main to move over to reach the correct throat. If there isn't anything that could be called a throat properly, then just to reach the correct route where there might be several going in each of the main compass directions to destinations spread around the region.