How comes, 100 ton and 125 ton trucks?

[bengt]

American freight car trucks are named 100 ton and 125 ton trucks.
As I understand 100 ton trucks are under 286 000 lbs cars and 125 ton trucks under 315 000 lbs cars.
But 286 000 lbs is 143 ton (metric 129,8 ton) and 315 000 lbs is 157,5 ton (metric 143 ton).
How can the trucks than be labeled 100 ton trucks and 125 ton trucks?

[Allen Hazen]

Possibly the weight of cargo as opposed to the weight of the loaded car?

[Eliphaz]

Two trucks under each freight car..?

[Allen Hazen]

Wasn't sure when I posted yesterday, but I've checked since.
(One place I checked: http://en.wikipedia.org/wiki/Hopper_car" onclick="window.open(this.href);return false; )

When the car, or the trucks suitable for the car, are called "100 ton" or "125 ton," that refers to the nominal payload. The maximum weight (315,000 pounds for a top-of-the-line unit-train coal car, 286,000 pounds for a goes-most-places-on-the-North-American-network car) is the sum of the payload and the empty weight of the car itself. (So a modern coal car is impressively efficient: it weighs 65,000 pounds (= 32.5 tons -- short tons, what is usually in question in the U.S. when you are not talking about ships...), but carries almost four times its own weight in coal.)

North American freight cars are stencilled with some basic data (white lettering in a box low on the side near one of the ends). Two lines relevant are
LT WT (the tare weight: 65,000 pounds in the example), and
LD LMT (the maximum payload: 250,000 pounds here).

(Pity about the continued use of Anglo-American traditional units, but Americans are VERY conservative, and getting them to adopt metric units has been given up as too hard!)