Layout design analysis
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list Sunday 11 June 2006 by joef
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Operation Potential Stats

Analyzing the operational potential really helps you refine your designFrom the basic stats, we can quickly estimate the layout's operating potential. Way back in June 1968, MR published "Layout plans by formula", written by Roy F. Dohn. Mr. Dohn described how to estimate the operating potential of a track plan using some clever formulas he developed by working backwards from actual operating model railroads.

Using his formulas as a starting point, I have developed an updated set of formulas.

MAXIMUM NUMBER OF CARS: A layout can only hold so many cars before you become unable to move because even the destinations are full. This upper limit seems to be around 80% of the total capacity for stationary cars, so we can compute this as: 80% of (storage + staging + passing/2).

To allow for more cars on the layout, increase the amount of storage and/or staging track, or to a lesser degree, add some passing track capacity. Generally, passing trackage is not intended to be used as permanent storage, so to indicate that some passing siding capacity could be used as short-term storage, a factor of one half is suggested in the formula.

NUMBER OF CARS MOVED: The number of cars moved in a typical operating cycle can be computed as: 40% of (staging x 2 + passing + connecting). To increase the number of cars moved, we need to increase some combination of staging, passing, or connecting trackage. Notice staging is particularly effective in increasing the number of cars moved, since for every train that leaves staging, another can move in to replace it, meaning TWICE the cars can be moved (if they are available elsewhere on the layout). In effect, staging acts as both connecting track and passing track -- thus serving double duty.

Another thing we can do to increase cars moved is stop using some track for storage, and designate it instead to be either staging, passing (if trains can legitimately "pass" on this trackage), or leaving it undesignated and always free of stored cars (so by default it becomes connecting track).

TRAINS: We can divide the number of cars moved by our average train length to arrive at the average number of trains we can expect in a typical operating cycle. Average train length is the smaller of average passing train length or average staging train length.

One operating cycle is defined as running the layout in a realistic manner until the trains you run begin to repeat. Ordinarily this will be one "24 hour" day according to the modeled train schedule. Depending on our fast clock ratio, the experience of our crew, the reliability of our equipment, the length of a typical run, and the level of detail to which we simulate prototype operating practices, the actual time it takes to complete one cycle could vary from one hour to dozens of hours. Three hours is probably a good typical cycle, however.

DISPATCHING THRESHOLD: Compute as (3 x shortest passing siding + 2 x average passing siding + longest passing siding) / 6. Two opposing trains of this size or larger will tend to create a dispatching bottleneck because they cannot easily pass each other except at select sidings. If you want to ease the dispatcher's workload, keep the typical train length at or under this size.

If you want the dispatcher to more easily manage longer trains, then lengthen your passing sidings. The best way to increase this threshold is to lengthen your SHORTEST passing sidings first. Of course, you need to keep the length of your staging tracks in sync with passing siding lengths as explained above under the train length stats.

Another less obvious tactic to improve this stat (if your passing sidings are smaller than your staging tracks) is to declare very short passing sidings to be switching runaround tracks only (and thus connecting track instead of passing track), thereby removing them from routine consideration as locations where the dispatcher might arrange meets. This tactic also has the effect of increasing the number of cars moved since it creates more connecting trackage.

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