The strange names of locomotive wheel arrangements
1Co-Co1 and A1A-A1A look like serial numbers. They are compact engineering drawings, and once you can read them they are obvious.
Few pieces of industrial notation look more impenetrable than a locomotive wheel arrangement. They are not codes to be looked up. They are descriptions, written left to right, of what is under the locomotive.
Letters and numbers do different work
Letters count powered axles. A is one driven axle, B is two, C is three, and so on. Numbers count unpowered carrying axles. Hyphens separate the bogies, or trucks, that the axles are grouped into.
So A1A-A1A reads as two bogies, each with three axles, of which the outer two are powered and the middle one is not. That centre axle exists to spread weight across more of the track, which matters on lightly built lines.
1Co-Co1 follows the same rules. Eight axles across two bogies, six of them driven, with one unpowered axle leading each bogie for the same weight distribution reason.
Why the notation survived
Because it answers the questions an operating railway actually asks. How much of the locomotive's weight is available for traction, and how heavily does each axle load the track. Both are readable directly from the arrangement without opening a specification.
The vocabulary around it stayed equally practical. A collector shoe is the contact that picks up current from a third rail. A Rollbock is a bogie that carries a carriage built for one track gauge along a line of another. Each name describes the job rather than the maker.