Model Railroading > How many cars can a loco pull?

HO scale,,,for "realistic" operation,,how do you figure out how many cars a locomotive can pull?
i have the "Field guide to Trains" paperback book,,lists a SD40T-2 at 3000 hp,,is there a model railroader way to figure out how many 40\'-50\'-60\'-86\' ect boxcars? flats?,,i want to model intermodal,,how many 5 cars setups could a single 3000hp locomotive pull?,
I have my cars wieghted to the NMRA standards,,,is that wieght full or empty? or does it mater?,,i guess what im trying to ask ,,is there a formula to say a
40\' loaded boxcar would be 200hp?,,something like that,thanks Dave

The two important questions are how fast do you want them to run and how steep is the grade.

One unit can easily pull 100 cars on the flat if you only want it to go 10-12 mph. Put it on a 2% grade and a 3000 hp unit can only pull about 6-8 cars at about 15 mph.

Western RRs usually rate their trains in Horsepower/ton. Hotshots get about 4 HP/T. Priority freights get about 2 HP/T. Drag freights and unit grain/coal trains rate about 1 HP/T. Some districts with rougher terrain might get another .5 HP/T. Helper districts get helpers to double or tripple the HP/T though the hotshots can usually climb 2% grades with 4 HP/T at 20 mph.

On a layout the locomotives can usually pull more cars than you can conveniently operate on the layout.

A loaded coal/grain car weighs about 143 tons.
Boxcars, tanks and covered hoppers vary from 70 to 130 tons.
Five well stack cars are about 360 tons loaded but count as 5 cars for HP/T. Empty they go about 120 tons.

That should give you some ideas.

I too model Intermodal stuff. What I do is 1-1.5 locos (depeneding on locomticve size) per 10 cars. I have a 21 car Intermodal, and it looks right with three engine consists with smaller power (SD60M, 8-40B, and SD45 is the consist I run it with), but at the same time, 2 SD90/43MAC\'s can pull it just as easy and it still looks right.

I also wieghed my cars down to NMRA standerds when loaded so the wieght is in the load.

Hope this helps.

Warren

We have 2 problems here. One is that there is no such ting as scale pysics. In other words, its impossible to recreate the same effects of gravity and friction have in the real world in simply scaled down form. Secondly, Model locos don\'t have different motors for different applications. The same motor powers everything from a 1500 hp F7 to a 6000 hp SD90MAC. I model the 50\'s so I\'m limited to mostly 40\' cars. More for convenience sake than anything else, I use a formula I read in MR (I tink) years ago that suggested 2 cars/driven axle. an A-B set of F7;s then pulls 16 cars and a Mikado pulls 8. Yeah, these are less than prototypical lengths, but given the compromises we have to make as to length of runs, it works for a layout based in earlier times. In modern times, layout design needs to focus more on longer runs to make a train of stacks look right. Most small layouts have a hard time because either the train takes up the entire layout, or you have a one car consist. Bummer.

I have a P2K SD-60 that can pull 50 cars on the level with NO slipping. I also have an Athearn SD-45 with a Hobbytowne drive that can pull 45 cars before it starts slipping. It all depends on how well weighted the unit is and the quality of the motor.

it can also depend on a few other things, physics being what it is....

My Athearn Mike couldn\'t drag 4 cars up a 2% until i removed the spring under the trailing truck. The spring actually lifted the rear drivers and reduced the pulling power. Now it pulls a dozen or more up the same hill.

On a steam locomotive, any sprung trucks, leading or trailing, will subract from the tractive effort of the drivers.

Physics...what a bear...

NMRA car weight is close to the weight of a loaded car. If you divide the prototype car weight with the cubic of the scale you model in you have the scaled weight. Ex H0: 120 ton/87x87x87=120/658503=0.00018 ton.

My H0 mordern diesels can pull 30-35 NMRA weighed cars each (on my layout). But a lashup of four locos can pull over 120 cars, but in sharp curves the side force can make cars tip over. A train over 80 - 90 cars have problems in curves of 30" radii or sharper.

A train of three locos at point and 18 to 20 cars look ok (to me) and is practical to operate (despite prototype have three time the numbers of cars).