Prior to the 1881-3 design of the Pennyslvania's Class K 4-4-0 American Standard steam locomotives (later reclassified to the Pennsylvania's Class D6 4-4-0 American Standards), most fire boxes were located between the frames of locomotives, restricting their width, and thus, the power of the locomotive.
PRR S1 | PRR L1s | PRR K4s | PRR E6 | PRR GG1 | PRR MP54 | PRR E44 | PRR DD1 | PRR D6 | PRR 3750 | PRR 1361 |