Two-digit road numbers

Why do some railroads give two-digit running numbers to certain locomotives? For example: http://www.jeffstrainsite.com/railfan_pics/Class_One/CSX/locomotive/csx_ac4400cw_28.jpg

They may do this for many reasons including:

1) They only have a low number of units and therefore give them low numbers like 10, 20, 30.

2) They are a big company like CSX (your picture) and they use various number-series to represent models. Let's say for example, the GP40-2s are number in the 10s, and they order 20 units. Well they'll have units number 10-19 (zero inclusive), making all 20 units in that series.

3) They may use a particular series for specific task or job like all units used for MOW may be numbered in the 20s, where as all units number in the 4500 series are used as road freight units, and are assigned to a southern mountain region.

John

Some companies even use single digits.
http://www.hebners.net/amtrak/amtGEN42_1_10/amt1fL.jpg

Usually all the same type of locomotive (say P42s in Amtrak's case) get assigned a range of numbers. They pick the range based on where there is enough space for all the engines their ordering to get #s in that range, without another type of engine being in the middle. Amtrak's P42s are all numbered in the 1-207 range, it allows you to know what type of engine it is, with only the number & operating company.

peter