First and foremost...

A little insight to Prototype Power Desk operations. So far on every model railroad I've had the joy of operating on, locomotives have either been permanently assigned to a specific train/job or the engineers were to pick any engine or pre-paired set of engines sitting anywhere on the layout.

However, from what I see on today's Union Pacific, which is what my layout will be based on, once a train has been built in the yard, the Power Desk assign engines based on a number of variables. One factor is tonnage, another is ruling grade between origin and destination, and one other is average speed.

Since a layout has defined end points with a set ruling grade between them, I thought I would concentrate on the tonnage (tractive effort) and speed (horsepower).

Tractive Effort

I used the following graph to determine what would be an appropriate amount of tractive effort to pull a full length train on my layout.

I didn't want to over complicate the math needed by the Power Desk to select locomotives, so here I use powered axles to represent tractive effort. The more axles an engine has, the more tractive effort it probably has. Yes, I know engine weight plays a big part in this also, but my goal is to have a good representation, not an exact replica, of power desk operations on today's UP.

The other thing that needed to be decided was the number of cars that would make up a train. I know a guy whose layout routinely runs 60 car trains. The size of a train is largely dependent on the size of the layout and its capacity. So I decided that on my layout, I could fit a 24 car train into my yards, and still have that look of a big 130 car coal train. 

If 24 cars on the layout is equal to a 130 car train on the prototype, and I think 3 AC4400 or SD90MAC's look appropriate (which is a total of 18 powered axles), then those are the 2 numbers I use on the first graph. The image above shows the powered axles in the column on the left, and the number of cars on the row at the top. Draw a line, and where that intersects gives you the amount of tractive effort needed for that number of cars.

I grayed out the areas on the graph to help me in moving the information to my Power Desk Chart.

Horsepower

Train cars get heavier the more lading gets placed inside them. So to only use the tractive effort graph to select the number of engines for a given train, wouldn't seem right for a train with 24 loads, or 12 loads and 12 empties. That's where my horsepower graph comes into play. Again, since the maximum train length is 24 cars, then obviously I could only have a maximum of 24 loads. But what if I had a train of 24 empties. On this graph, a loaded car = 1 load, and an empty car = 1/2 a load. so 24 empties is = to 12 loads.

I figured 12,000 horsepower would be a good round number for my loaded coal trains (an AC4400 has 4,400 hp, and on the coals trains I see here 3 engines are pretty standard, 3x 4,400 = 13,200, so a little excess).

Doing the same as before, I made the graph and drew a line. 

So there you have it

the way I see it, if the yard master builds a train, and goes through the car cards to see how many loads or empties, and how many total cars, he can give that information to the power desk. The power desk in turn will see what is ready on the fuel pad that meets the total number of needed powered axles and horsepower, and assigns it to that train.

So you may need 6,000 horsepower, which 2 GP40-2's could handle, but then see that you need 10 powered axles. 2 GP40's only have 8, but 2 SD40-2's have 12, so bingo, there you go.

That's the end of that, so feel free to comment.

Steven

Prototype info

First of all, most railroads (as far as I know) use horsepower per ton.  NS uses powered axles (limit 24 for most trains, but the formula is a little complex) and tonnage ratings over ruling grades.  So your plan to use powered axles is prototypical.  But I would argue that an empty should rate as 1/3 of a load, rather than 1/2.  A typical empty these days weighs around 30 tons.  Loads are between 100 and 130 tons.

Very interesting

Neat concept Steven.  I think it's interesting to see how people approach this according to their prototype.

I suppose I should ask

How many people replica the hostler/power desk job during operating sessions?

Again, all I've ever seen on the many layouts I've operated on, it's just a free for all of the engineers picking their engines.

Do you have sets of MU'ed engines, or does the job of the hostler entail consisting, un-consisting, and assigning power to trains throughout the session? 

Not here

Sorry if I misunderstood Steven.  My layout's not busy enough to justify a dedicated hostler position, but neither do crews pick their own power.  I model an out-and back operation that leaves staging, runs west to my major yard at Council Bluffs on the other end of the layout, and then, next session, returns east to staging.  As a result, power decisions are made by the trainmaster (me) before the start of session 1.  Since the power off the westbound will bounce back east at Council Bluffs, I have to take into account the eastbound tonnage coming from other roads, industry pulls, and the ramp there, and assign appropriate power to not only get the train to CB, but back out again.