According to my brother who is an engineer - DP makes it harder to keep
a train in one piece. Just when you want to strectch it out the tail
end is bunching it up and vice versa. The engineers hate them.
Steve Hosk> On 11 Aug 2003 01:07:45 GMT, firstname.lastname@example.org (CBT2000) wrote : >
Precisely. Coal trains max out on weight. Coal hoppers don't often move
with less than rated load (unless empty of course). That's not true of
many other commodities that may 'bulk' out (fill the car) long before
maximum weight is reached.
Most cars today have capacities on the order of 100 tons (70 tons in
W.W.-II days). That's approximate. That's the most that can be carried
on eight (freight car sized) wheels. Approx. 25 tons per axle, 12.5 tons
per wheel. The problem lies with the rail, not the wheels. Exceed the 25
tons per axle and you permanently deform the rail. A LITTLE of this can
be tolerated (as with locos), but if all the cars in the train exceed
this limit, the rail is rapidly destroyed. Attempts to run 125 ton cars
have made this problem quite obvious. Bigger wheels can carry more
weight without damaging the rail, hence the tendency towards 36" and
even 38" wheels on some equipment.
Similarly, six axle cars can carry on the order of 150 tons. The extra
upkeep cost on the more complicated trucks, and tracking problems,
limits their use. Similarly for four (or more) truck cars. They're used
where they're NEEDED, but not in mass numbers.
To use the example previously stated: A loose feather car would have to
be positively HUGE to contain 100 tons of such feathers. Far bigger than
a Hi-Cube boxcar. Clearances prevent such a monster, so bulk feather
cars [ *IF* any :-) ] run at WAY below the 100 ton limit. They 'bulk
out' long before they reach maximum weight. Much mixed freight does this
Plus mixed commodity trains will usually have a mix of loaded and empty
cars. Bulk transport cars will often run in sets (sometimes unit
trains), and will be either all full, or all empty. We're not
considering the 'empty' case here.
And, a railroad may add more cars to a train if the cars are lighter,
but there are limits on siding length, etc. that limit this practice.
Such trains will not usually max-out on weight ... something else will
limit train size.
SO, loaded coal, ore, and similar trains are, on the average, heavier
than loaded mixed commodity trains.
"Gregory Procter" <
Shovelling seven tons in a 10 hours shift into a firebox. Been there, done
there, got the T-Shirt.
You can do that because each shovel full doesn't weigh that much.
Agreed, but compared to what? For a given volume, coal is light compared
to iron ore and heavy compare to feathers.
Look at cars designed to haul a particular commodity. If the car is
large, the commodity has low (weight) density. Such cars often 'bulk
out' before they reach their weight limit. If the car is small, the
commodity has high density. such cars (loaded) will usually be AT their
weight limit. That's why ore cars are so much smaller than high-cube
boxcars ... they both have about the same weight limit, something like
70 -100 tons.
Coal hoppers are sometimes used to haul iron ore ... they just can't be
filled only about half full or the car goes overweight. Seen from the
ground they look like 'empties', but look at the truck springs and you
can tell they're loaded to (weight) capacity.
"Roger T." wrote:
> > SO, loaded coal, ore, and similar trains are, on the average, heavier
> > than loaded mixed commodity trains.
> Agreed. But the original poster stated that coal is heavy, which it isn't. > :-)
> Roger T.