I know in this wacky world of ours we can find an example of almost everything. Speaking in generalities though, do modern railroads limit train lengths to accomodate passing siding lengths? Are some trains too long for some sidings thus giving them priority (in dispatching) over shorter trains that will in fact fit in the siding? Does this question make sense?
Length of passing sidings
Length of passing sidings
Tom C
Yes, the question DOES make sense, so, FWIW, my few cents:
Do modern railroads limit train lengths to accommodate passing siding lengths?
Yes, because train still have to overtake each other (Pass), or make opposing movements (Cross) as one train comes down a specific length of track and wishes to go onto a section which may already be occupied by an opposing movement.
Crossing Loops (US-Passing Sidings) are placed to allow these movements to occur and are finite in length, the length being determined by a number of factors including geographical location and available space on site.
Are some trains too long for some sidings thus giving them priority (in dispatching) over shorter trains that will in fact fit in the siding?
Again the answer is 'Yes', longer trains do tend to have priority when moving through a specific section. However, this must be qualified , as the longer trains tend to be slower and more difficult to accommodate if something goes wrong.
Invariably the smaller (shorter) trains DO have to take the Crossing Loop to get out of the way of the longer unit, a fact which tends to play havoc with scheduling and cause the Train Controller (Despatcher) to divert the smaller ('faster') units out of the way so that the larger train can get through.
Shorter trains that fit within the available Crossing Loop spaces are far easier to work with as they can be 'threaded in and out' between each other and moved through far more quickly.
Hope this helps. Thanks for asking.
Komata
"TVR - serving the Northern Taranaki . . ."
When building a passing siding...
...for a model railroad, conventional wisdom says to plan the siding to accomodate the longest train you anticipate. However, it is not the end of the world if you want to run a train longer than the passing siding. There is a maneauver that will allow trains longer than the siding to pass each other. It is called a SAW-BY movement.
In the first instance one train is longer than the siding. The second train is shorter than the siding. If both trains arrive at the siding at the same time or if the shorter train arrives first, the shorter train goes into the siding and lets the longer train procede on the mainline.
In the event that the longer train arrives first, the saw-by movement starts when the longer train pulls into the siding and stops short of the exit points with some of its cars still on the main line. The shorter train then goes forward on the main line until it clears the exit points and then stops. The longer train proceeds forward and returns to the main line. After it has cleared the entry points the shorter train continues on its way as well.
In the event that two trains meet at a siding and both trains are longer than the siding, a DOUBLE SAW-BY maneauver must be done.
1. The westbound train drops off a number of cars on the main line so that the remaining train is shorter that the siding. It then moves forward until it is between both sets of points.
2. The eastbound train then proceeds through the siding and couples the dropped off cars to the front of its locomotive.
3. By this time the westbound train must be at least 1.5 train lengths down the main line from the siding.
4. The eastbound train then backs into the siding and drops the cars off the front of the locomotive, leaving them on the siding.
5. The eastbound train then continues backing until it is once again on the mainline. It then proceeds on its way eastbound.
6. The westbound train then backs into the siding and recouples its cars. It then can go on its way to the west.
Have fun.
Moose
Length of Passing Sidings
Moose
Thanks for the description o0f a Saw-By - fascinating.
I would suggest however, that this is a train movement that, if it is still being used, is almost unique to the United States, and rare outside that country, as the Mechanical Interlocking, of Points and Signals, especially where coloured light circuitry is involved, tends to make such a movement very difficult to achieve - in the normal state of affairs Signals (both semaphore and coloured-light) combined with Mechanical-interlocking, firmly discourage such moves, as being of an unsafe nature.
I had the misfortune (my description) to participate in such a movement in a situation where all signals and electrical systems were non-operational because of a weather-related event. Suffice to say that it was not pleasant, as it involved hand winding of points and the use of flags and a lot of walking - not a scenario that I would care to repeat - very messy, very time consuming, and without benefit of R/T, a very long process.
Undoubtably the movement will still occur on many railways, and was probably very common prior to the introduction of Fixed Signals and Mechanical -interlocking of points and sidings, but as a 'normal' state of events it would, I suspect be very rare indeed. If it still exists as a normal part of US RR operation, I can only presume that the CTC sytems in use have been adapted to suit the situation, and that there must be provision for the use of uncontrolled (non-interlocked) sidings within the CTC-environment which will permit such movements.
But again, thanks for the description.
Komata
"TVR - serving the Northern Taranaki . . ."
One other thing to keep in
One other thing to keep in mind is that passing sidings limit the length of a train in one direction only (unless of course a faster train needs to take a slower train). If you keep all the traffic operating in one direction to the length of your sidings, you could operate a train in the opposite direction (or one running faster than all the others in the same direction) any length you please, as it will never take siding.
Or possibly you could run long trains trains in sections, depending on your era. I am not sure if the prototypes do this anymore or not...
Oh, I totally agree,
Oh, I totally agree, Moose. I think everyone should challenge their operating skills by running long trains once in a while! 
But, what is fun once in a while could (possibly) get quite old in constant practice on a model railroad, so that should be a consideration in design.
Just my two bits!
Joined: 2004-09-08