The design of watertube boilers, which are commonly the ones used in Marine and Naval service, requires this sort of arrangement (on later, oil fired boilers, the BT's would cut in additional burners to allow a faster steam generation rate when required.) Locomotive boilers, on the other hand were firetube boilers and not watertube (a few railroads, most notably the B&O, tested hybrid water tube boilers, but they were not successful). The main reason firetube boilers remained in use on steam locomotives, was that the large reservoir of water in the boiler would allow the boiler to meet a changing steam demand faster than shoveling more coal in the firebox could (although increasing the firing rate would allow the boiler to "catch up" to the increased steam demand).