Locomotive numbering - Model Railroader Magazine

There is no one answer to this question that applies to all railroads, or even to the same railroad.

But for most railroads, locomotives (steam and diesel) came in classes and those classes would have a number.  So for example there might a "4000" class consisting of 25 engines:  Number 4000, number 4001, number 4002 and so on.  Sometimes railfans mention getting a photo of "the class engine" -- the lowest numbered of the class.

Now imagine a railroad that had a 4000 class of engines, and then a 4100 class of entirely different engines.  If it happened that they acquired "too many" of the 4000 class they might have to renumber or invent new class numbers.  Many steam locomotives, and the Milwaukee Road is a particular example of this, renumbered their engines many times during their service lives.  Thus a real exacting Milwaukee Road modeler with a specific era in mind might have to repaint or renumber a model that is otherwise perfect for their layout.

Some railroads were small enough and acquired so few engines that the number system was keyed to when they got the engine, so for example an engine acquired in March of 1945 would be 345.   What if they got more than one engine in the same month?  Frankly I do not know but i gather the kinds of railroads that used this "system" tended not to have that problem.  By the way some model railroaders with their own layout name use that same system to keep track of when they got an engine.

The Pennsylvania Railroad was an example of a huge railroad with distinct and very precise classes of engines, but a numbering system that at time seems purely chaotic.  I have read that there was indeed a system of some kind to the numbering, but I have never read an explanation that I could actually track (no pun intended) in my mind.

One problem of course is that a truly huge railroad might soon run out of numbers.  For a variety of reasons four digits seems to be the max.  In the case of the Union Pacific they have arrived at the clever solution of having UP and UPY reporting marks -- invent a new reporting mark and you can start all over again on the numbers!   Some railroads would assign engines to subsidiaries, some of which were "paper" railroads otherwise not using that name much if at all.  But again that would open up a fresh set of numbers.

Talking about the B&M, this useful website

contains a chart of numberings for their four classes of 4-8-2:

4-8-2 Mountain

So as you can see, 4105 is in a sense the "class engine" for the R-1b class.

Dave Nelson