Discussion relating to the PRR, up to 1968. Visit the PRR Technical & Historical Society for more information.
  by AllenHazen
The PRR classification system for steam locomotives that we are all familiar with -- K for Pacific, L for Mikado, etc, with basically a letter for each wheel arrangement -- was introduced in the early 20th Century (exact date?): there had been an earlier system in which different letters were sometimes used for locomotives of the same wheel arrangement but different driver diameters (I think). Now, the allocation of letters in the modern system seems fairly systematic... but with a few anomalies: letters skipped, at least temporarily. Would these have been letters used in the older system for 19th century types with a few surviving members?
A: 0-4-0
B: 0-6-0
C: 0-8-0 (but the C1 wasn't introduced until later: perhaps the people composing the new system thought the PRR might acquire some 0-8-0 switchers, and so left "C" available, so all the switcher types would have class letters in the same range?)
D: 4-4-0 (I don't think many 4-4-0 were built after the new system was introduced, but there were a lot still in service)
E: 4-4-2 (the next step in passenger power...)
F: 2-6-0 (a standard turn-of-the-century freight type)
G: 4-6-0 (the memorable G-type in the modern period, the G-5, was later, but there would have been lots of older Ten Wheelers around)
H: 2-8-0
Anomaly. I: 2-10-0 (The I-1 wasn't introduced until the early 1920s. Why was the letter I skipped in the early 20th C? Fear of confusion with 1? Too many locomotives of some older I class still around?)
J: 2-6-2 (This is systematic: a new wheel arrangement introduced in the period of the new classification system gets the first (give or take I) letter available)
K: 4-6-2 (Next new type)
L: 2-8-2
Anomaly. M: 4-8-2. (The M-1 wasn't introduced until after the N classes -- N-1 and N-2. So why wasn't "M" used for Santa Fe?)
N: 2-10-2 (See comment on M)
O: 4-4-4 (Canadian Pacific had steam of this wheel arrangement, but PRR evidently felt no temptation to acquire any -- perhaps because the still had many very good E-6 Atlantics to cover any runs a 4-4-4 would have been good for -- and so used "O-1" for the first electrics of this wheel arrangement)
P: 4-6-4 (They may've thought they MIGHT get some steam Hudsons -- the K-5 Pacific was an experiment that, in the end, didn't merit series building -- and so called their first electric 4-6-4 the "P-5": skipping to 5 had a precedent, since the first 2-8-2 electric had been called the "L-5")
Anomaly. "Q" was later used for the 4-10-4 duplexes, but was skipped in the 1930s.
R: 4-8-4 (I would have expected "R-5" for the 2-D-2 electric instead of "R-1", but maybe by the time it was ordered they weren't anticipating any steam Northerns? If they had later acquired some -- serious consideration was given to the type a couple of times -- would they have gotten the number 5 in their class designation?)
S: 6-4-4-6, and later 6-8-6. (Maybe they thought coupled and divided drive variants weren't different enough as wheel arrangements merit different letters?)
T: 4-4-4-4 (Though, if coupled and divided drive variants could count as the one wheel arrangement for purposes of class designation, these COULD have been called "R-5".)
Most of the letter choices make sense: order is roughly chronological, with larger types getting later letters for the types already in service when the system was devised. (I think the two experimental 2-6-2 J class were scrapped early enough that "J" probably could have been re-used a bit earlier.)
I should check this, but I think the old H-3 Consolidations had been "I" class under the old system, and there might have been enough of them still on the roster (long demoted to switching service, I suppose) to explain why that letter was skipped when the first Prairies and Pacific were ordered.