Re: Freight train speed on grades - early diesel era


MDelvec952
 

In a message dated 12/30/2006 3:30:18 PM Eastern Standard Time,
mlaughlinnyc@... writes:




I'm involved in a discussion/debate about the speed of freight trains on
grades in the 50's. One of our members contends that 10 mph was a typical
freight train speed on grades. While this may have been true on short grades in the
steam era, it doesn't make sense to me on long grades in the diesel era.

My understanding of the mechanics of the situation is that a diesl
locomotive suitable for hauling a freight train several hundred miles at normal speeds
could not have run at 10 mph on even ten miles of grade without burning up
the traction motors.





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Two of the many reasons diesels replaced steam were 1: the m.u. cable, and
2: the fact that diesel-electrics' efficiency gets greater the slower the
speed, versus steam which acheived its greatest efficiency the faster it ran,
ideally at its balancing speed (function of driver size, stroke, cut-off and
counter-balancing). Much of freight railroading takes place at less than 20
mph, and it was here at the diesels really outperformed steam. New York
Central and N&W and PRR conducted tests, and the NYC found that the Niagras at 50
mph cost less to run than diesels, and N&W combined design with streamlining
its steam maintenance to make its 50 mph freight trains with Class As, and
passenger trains with the Js, more cost-effective than diesels. N&W held out the
longest, but it still remained that the overall cost savings of the diesels
was greater than the financing costs to purchase diesels.

You want to move a given tonnage at a higher speed, keep adding locomotives.
The m.u. cable made that financially practical. A single diesel-electric
can start a full-tonnage train, but won't move it very fast. (Add enough
diesels and one crew can move that tonnage as fast as you want.) Steam locomotives
had their biggest struggle in starting a train, if a steam locomotive got a
train moving on flat track, it could accelerate with it (ideally to the
machinery limits). A diesel will accelerate up to its short-time ratings, and
that's all the speed you'll get until more diesel-electrics are added.

Steam tried to win its starting-a-train battle with booster engines that had
to be disengaged once the train was moving. I was onboard one steam
locomotive with a booster engine, and it sounded like it was doubleheading with a
Shay -- neat stuff, on Frisco 4-8-2 1522 starting a 12-car passenger train on a
grade at Mukwanego, Wisc.

To get more toward your discussion, speed was not a limiting factor on
diesel-electrics, but the ameter was. Ideally a diesel-electric can run uphill at
5 mph as long as it has fuel, provided the current to the traction motors
doesn't exceed the short-time ratings. One diesel might red-line moving a
5000-ton train at 5 mph, but four diesels running together would not, and would
purr along happily and unwinded at 5 or 10 or whatever.

All railroads somewhere had a preferred horsepower-per-ton ratio they wanted
on certain territories, and often this was in the employee timetable. Early
in the diesel days everyone was still learning, so these may have been in
bulletin orders until finally being added to the timetables. That number often
changed depending on the locomotive. I remember reading somewhere that SP
wanted 4 hp per ton over the Tehachapis; some of the earliest diesel
streamliners at 600-1000 h.p.. generated 6 hp per ton of train weight, which enable them
to run at 90 mph.

Baldwin, for example, used a main generator that wasn't really capable of
generating enough current to damage its traction motors. So an engineeer would
open that air-operated throttle and sit back and wait for his train to move. An
d it always did. Talk to any old-timer who ran those first Baldwins and
he'll mention his complaints, if he had any, and wrap it up with "but it would
pull anything," or explatives to that effect.

Diesels intended for slow speed drag freights were ordered with a gear
ration more suited for that service, similar to steam in that regard (small
drivers, big pistons, long stroke, fat boilers)

The difference in our thinking about diesel-electrics and steam is that word
"electric." All piston engines are happiest near their balancing speed, but
in a diesel locomotive traction is provided by an electric motor, and only
so much current can travel through a certain size wire before that wire heats
to hits melting point., the weakest link generally being where the brush
contacts the armature. That problem all but went away with the a.c. traction
locomotives in the 1980s when engineers were told they could sit on a grade
stalled in run-8 and not do any damage, since there was no brush touching an
armature. For that reason, and a computerized wheels slip system, those first
SD60MACs achieved a never-before-heard-of 170,000 pounds tractive effort, more
than half the locomotive's 300,000-lb weight got converted to adhesion for
the first time in history. Today's modern miracles of adhesion have hit their
limit at 4000-4400 hp, but that limit was the metalergy of the steel wheels
and rail -- the wheels were failing at 6000 h.p. and the modern wheel slip
systems. The goal with steam was a factor of adhesion of 4, 25% of the engine's
weight in drawbar pull.

Back to pistons -- Diesels engines love to run hot and hard, near their
balancing speed, or rpm. EMD used to test its engines by installing them in a
power plant at the LaGrange factory. I was there once and saw rows of diesel
engines, and they'd run at Run-8 for weeks and months at a time when the
engineers would take one out systematically to check wear and bearings and
lubrication, etc. These engines were tied to generators that would supply
electricity to much of the plant.

Real life: I'm a trainmaster for a short line responsible for our biggest
customer, a refinery. I have to argue and bargain for various locomotives
against the needs of the rest of the company when ours have to be shipped back to
Morristown for maintenance. We bring in and put out 30-50 cars each day
with lots of in-plant switching. All of our running is at 10mph or less on
hairpin curves and 4% grades. I like the Alco Century 2400 hp C424s on our
roster, not because of the horsepower as much as the weight-on-drivers and the
3000-gallon fuel tanks, which add weight when full. This weight makes them sure
footed so the crew can get its normal work done in 8 hours. About a year ago
Morristown sent down an RS11, 1700 hp, 1500-gal. fuel tank, pre-Century
electricals. Turned out it is a great puller, but slippery at times (due to the
lighter weight) and with a running start can pull / shove the heavier trains.
Two months ago they sent down an SW1500, 1500 hp, 1000 gal. fuel tank. Moves
trains, but is slippery (doesn't weigh much), but can lug surprisingly well.
But it gets close to its short time ratings and the slower pulling gets us
into overtime with the crews. In our biggest yard, 18 tracks, longest is 20
cars, digging out cars takes forever compared to the heavy road units.

So, to answer your questions:

Freight speeds early diesel era: Depended on track speed and the number of
diesels in the train.

On long grades: Same answer.

Short time ratings on diesels have much more to do with traction motor
current than running speed.

Ten mph may have been the typical speed noted, but the combination of
deisels and tonnage may have often occurred creating 10 mph, or it could have been
track speed, or both, If the track speed was 10, then that's all they did.

If a train didn't have enough diesels, or one shut down during the pull, the
train slowed down, and sometimes the crew would get into short-time trouble
and have to stop.

Dispatcher sheets: Station times are good, but early in the diesel era
there was still a mentality of using helpers, so some of your times would include
the adding and dropping of helper sets. It today's world helper districts
are getting scarce -- its easier and cheeper to put enough units on most trains
to make the helper districts invisible.

Mike Del Vecchio

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