Reposted here is George Drury's text of how he came up with the cataloging system Kalmbach Publishing used in its library, and may still be using today:

Drury Non-Decimal Library Classification

The David P. Morgan Library is so specialized that it would be able to use only a small portion of the Dewey Decimal or Library of Congress classification systems. The classification system is based on the collection itself and the way it is used. When I made it up I tried to be mnemonic (those words in brackets in the list below) and also follow the instructions I got: “Keep things where they have always been so we can still find them, but make it better.”
Items are shelved by major class, then by “cutter,” usually author or date.
For example, Bill Middleton’s recent book on Penn Station is coded
S
Penn
for specific railroad, Pennsylvania. His book The Railroad Scene, a
collection of photos covering most of North America, is
R
Mi
for railroads, general, Middleton. If Middleton were to write a biography of
Frank J. Sprague, it would be
B
Sp
for biography, Sprague. The 1940 Car Builder's Cyclopedia is
CC
1940
Other cutters appear in parentheses in the list below. Oversize books have a Z prefixing the code the books would normally get. They are shelved on the top shelf more or less above where they would be if they were smaller. An underscored code indicates the item is in a shelf file box (pamphlets and other small items that would get lost on the shelf). A yellow tag indicates the item is not owned by the library, and the catalog record is so noted.

A Aviation and other transportation
AB Boats
AM Military
AS Space
AT Transportation, general
AV Vehicles cars, trucks, buses
B Biography (biographee)
BS Speeches (speaker)
BW Who's Who (date)
BWR Who's Who in RRing (date)
C Cars
CB Carbuilders
CC Carbuilder's and Car & Locomotive Cyclopedias
CF Freight cars
CM Mechanical
CN Non-revenue cars
CP Passenger cars
D Operation [Doing]
DA Accidents
DB Bulk commodities
DC Communications
CF Freight service
CM Mail service
DP Passenger service
DR Rule books
DS Safety
DW Military railroading [War]
E Engineering
EB Buildings
EC Engineering cyclopedias
EE Electrical
EM Mechanical
ER Bridges
ES Signals
ET Track
F Fiction, poetry, and music
FJ Juvenile
G Guidebooks
GE Geography
GR Railfan guides
GS Radio frequencies [Scanner]
GT Rail travel guides
H Labor [Hired Help]
HE Educational
I Government and associations [ICC]
IC ICC
IM Mergers
IP Proceedings
IR Reports of various kinds
IS State reports
IT USDOT
J The Railroad Problem
JC Competing modes
JP Passenger
JS Solutions
JT Technology
JU Urban transportation
K Economics and transportation
L Locomotives
LB Builders and builder catalogs (builder)
LC Locomotive Cyclopedia (date)
LD Diesels
LE Heavy electrics
LG Gas turbines
LR Railcars
LS Steam, technical
M Model railroading
MA Aviation and space modeling
MB General modelbuilding
MC Cars and other road vehicles
MD Miniatures and dollhouses
MDA Antiques
MDB Full-size buildings and architecture
MDC Costume
MDD Dolls
MDE Decoration
MDF Full-size furniture
MDH Dollhouses
MDM How-to books < houses, furniture
MDO How-to books < other
MDP Patterns
MDR Artisans and
their collections
MDT Toys
MG Gaming
MK Crafts
MM Military
MO Other modeling
MP Plastic techniques
MR Radio control
MT Tools and techniques
MW Boats [Water]
MY Model-building reference
N Toy trains
NA American Flyer
NC Catalogs
NG LGB
NL Lionel
NM Märklin
NT Toys other than trains
NX Marx
O Official Guides and timetables
OE Employee timetables
O/X Foreign
P Periodicals
Q Statistics
QA Annual reports
QC ICC reports
QR AAR statistics
R Railroads, general (author)
RA Railroadiana
RJ Juvenile
S Specific railroads (railroad name)
T Traction (geographic)
TA Traction (author)
TB Traction builders
TC Cable cars
U Museums
UL Preserved locomotives
UT Trolley museums
V Books not otherwise categorized
VI Books about video
VT Videotapes
W Writing and reading
WD Design and typography
WP Photography
WPC Photo catalogs
X Foreign (except Canada and Mexico)
XA Africa
XB Britain
XD Scandinavia
XE Europe, NOIBN
XF France
XG Germany
XJ Japan and Asia
XL Latin America
XR Russia
XS Switzerland
XU Australia and New Zealand
XW World, general and NOIBN
Y Reference
YA Almanacs
YD Dictionaries
YE Encyclopedias
YI Indexes
YM Maps and atlases
YR Railroad reference
Z Oversize

George Drury