They mean nothing.
To be more specific, they mean (or meant) whatever the locomotive maker
wanted them to mean. There were, however, some conventions followed:
GP = general purpose (called "geeps")
SD = special ... mumble mumble [something]
Welcome to trainspotting.
Yes. And the GPs are four-axle.
I understand the Geeps were the originals, and the six-axle SDs started out
as variants that were essentially the same locos, but the six axles gave
them lighter wheel loadings, and therefore they could run on lighter-railed
branch lines that the Geeps couldn't manage.
It was shortly discovered that dividing the same engine output into six
traction motors instead of four gave the SD models much more "oomph" at low
speeds, and they rapidly became favorites as mountain engines.
Modern day SDs have given up the idea of having a lighter axle loading
because of their humongous size. They need six axles just to distribute that
Am Thu, 07 Feb 2008 00:26:06 -0800 schrieb P. Roehling:
Not true. Most of the SDs were essentially 50% heavier than the
corresponding Geeps, for increased tractive effort. Only the SDL-39 built
for the Milwaukee Road was lightweight, and Alco had six-axle versions of
its RS-series with lower axle load. As a matter of fact, six-axle engines
are often not allowed on branch lines, as three-axle trucks put more
strain on the track than the two-axle trucks.
Could you cite that please? Everything I can find on the early SDs (7s and
9s) indicates that normally the only extra weight in them was a somewhat
longer frame, 6-wheel instead of four-wheel trucks, and a pair of extra
wheels and traction motors. That would hardly add up to 50% more weight than
the GP models they sprang from, as everything else on the SDs was the same
as the GPs.
Now there *were* some early SDs that were built with added weight: two of
the S.P.'s early SD7s (numbers 5288 and 5289) had an extra 15 tons of
concrete in their frames, but according to John Signor on P. 221 of his book
"Southern Pacific's Coast Line", "it made little difference in practise",
and I've seen nothing that suggests any more of the S.P.'s SD models were
ever ballasted after that.
Am Thu, 07 Feb 2008 23:19:39 -0800 schrieb P. Roehling:
The Fallen Flags website (www.rr-fallenflags.org) has the operating manual
of the SD7, which says that it was available in either 300,000 or 360,000
pound versions. SD18 is 386,460 pounds, SD24 between 330,000 and 390,000,
depending on the amount of ballasting. GP7 is 240,000 pounds. So yes, the
"light" SD7 actually has a slightly lower axle load than the GP7 (25 tons
vs. 30 tons), but most others are 50% heavier than the Geeps.
You also said in your original mail that modern SDs need the six axles to
distribute the weight, implying that a similarly powerful engine could not
be built light enough to use four axles. I don't agree with that, as there
were both GP60s and SD60s, both with a 3,800 hp prime mover. I don't think
it would be a problem to build a GP70ACe or whatever it would be called,
it's just that nobody would buy it, as they wouldn't be able to develop
the tractive effort necessary to get all that power on the rails. NS even
orders their new locos downgraded to 4,000 hp (ES40DC).
Go back and read my original post. I was speaking only of the early SDs when
I said "they *started out* as variants that were essentially the same locos,
but the six axles gave them a lighter wheel loading".
I said what I meant, and I meant what I said; so if you wanna play
nit-picking word games, go find someone else to bother.
Me, I'm anti-semantic.
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