Editing Whyte notation (section)

=== Articulated locomotives ===
For [[articulated locomotives]] that have two wheelsets, such as [[Garratt locomotive|Garratt]]s, which are effectively two locomotives joined by a common [[boiler]], each wheelset is denoted separately, with a plus sign (+) between them. Thus a 4-6-2-type Garratt is a {{whyte|4-6-2+2-6-4}}. For Garratt locomotives, the plus sign is used even when there are no intermediate unpowered wheels, e.g. the [[LMS Garratt]] {{whyte|2-6-0+0-6-2}}.  This is because the two engine units are more than just [[power bogie]]s. They are complete engines, carrying fuel and water tanks. The plus sign represents the bridge (carrying the boiler) that links the two engines.

Simpler articulated types, such as [[Mallet locomotive|Mallet]]s, have a jointed frame under a common boiler where there are no unpowered wheels between the sets of powered wheels. Typically, the forward frame is free to swing, whereas the rear frame is rigid with the boiler. Thus, a [[Union Pacific Big Boy]] is a {{whyte|4-8-8-4}}: four leading wheels, one group of eight driving wheels, another group of eight driving wheels, and then four trailing wheels. Sometimes articulated locomotives of this type are denoted with a “+” between each driving wheels set (so in the previous case, the Big Boy would be a 4-8+8-4). This may have been developed to distinguish articulated and duplex arrangements; duplex arrangements would get a “-“ being rigid and articulated locomotives would get a “+” being flexible. However, given all the wheel arrangements for duplex locomotives have been mutually exclusive to them, it is usually considered unnecessary and thus another “-“ is usually used.

[[Triplex locomotive]]s, and any theoretical larger ones, simply expand on basic articulated locomotives, for example, [[2-8-8-8-2]]. In the case of the Belgian [[quadruplex locomotive]], the arrangement is listed as 0-6-2+2-4-2-4-2+2-6-0.<ref>{{cite web | url=http://www.douglas-self.com/MUSEUM/LOCOLOCO/francocrosti/francocrosti.htm#b | title=The Franco-Crosti Boiler System }}</ref>