Every railroad has a numbering system that makes sense, at least within the corporation. They are not only neccesary to keep track of maintenance records, but simply by referencing the car # or series, every employee would instantly recognize the car type without need for additional explainations. Many freelance modelers, myself included, never really think about this - and just assign road numbers at random. While building a few recent projects, I started to question my 'whatever number is handy' philosophy. The following is my attempt to make order from chaos without having to renumber too many existing pieces.

AV Number System:
From now on this is the simple numbering system I intend to use. There are many more numbers than existing equipment to allow for future builds, trades, or purchases - and to better imitate the feel of a real railroad operation.
Steam Locomotives:
#1-6 Narrow Gauge Switchers
#7,8 Narrow Gauge 'Special' or Mixed Traffic Engines
#9-12 Narrow Gauge Passenger Locomotives
#14-19 Narrow Gauge Freight Engines
Note 1: Prior to 1874 locomotives were identified only by name. After that date they were eventually  numbered in order of purchase

#60-68 Standard Gauge Switchers
#80-95 Standard Gauge Freight Engines
Note 2: SG numbering was only formally adopted in 1945. Prior to this, the number of the unit it replaced, or the next available one were given
Note 3: There are no SG passenger locomotives - as the AV was narrow gauge, and later dual gauge during the entirety of passenger operations.

Diesel Locomotives:
# 40-45 Narrow Gauge Switchers
#46-49 Standard Gauge Switchers
#50-59 Standard Gauge Road units

Rolling Stock:
#21-29 Narrow Gauge Passenger Equipment
#31-32 Narrow Gauge Baggage
#33,34 Narrow Gauge Postal Cars
#71-79 Narrow Gauge Passenger Equipped Freight Cars
Note 4: See note 2 above.

#100-129 Narrow Gauge Tank Cars
#130-137 Narrow Gauge Flatcars
#140-175 Narrow Gauge Boxcars
#240-249 Narrow Gauge Caboose
#301-345 Narrow Gauge Gondolas
#401-499 Narrow Gauge Hoppers

#501-545 Standard Gauge Hoppers
#551-559 Standard Gauge Flatcars
#561-579 Standard Gauge Tank Cars
#601-645 Standard Gauge Boxcar
#650-659 Standard Gauge Caboose

An 'X' Prefix: Special Equipment - well cars, water cars, snow removal, fire fighting, cranes, etc.
#X01-35 Narrow Gauge
#X41-55 Standard Gauge

An 'M' prefix: Added to any number is to designates MoW service. Same numbering as for 'X', with the addition that  ex-revenue cars will simply have 'M' added to the old car number -  The original 2 or 3 digit number may then be re-issued to another revenue car. 
A 'B' prefix: Added to any number indicates the car has been assigned to in-house Business or Operating Department use

The Current Locomotive Roster:
"Nonesuch" 4-4-0 1856 Rogers


#2 1888 8-ton class A Climax


#6 0-4-0 1885 Porter


#8 2-4-4-2 1917 AV Phillipston



#10/ #4 2-6-0 1905 Baldwin, rebuilt as an 0-6-0 in 1940


#12 2-6-0 1918 Baldwin


#15 2-8-0 1922 Baldwin from 1917 Standard Gauge