On a steam locomotive, the throttle is conventionally mounted within the steam dome, in other words at the very beginning of its route from the boiler to the cylinder. Thus, already, it is far from the place at which the steam does its work. If a superheater is installed on the locomotive, this problem is compounded by the long lengths of the superheater elements. Because the point of control is so far away, the response of the locomotive to adjustments of the throttle lags substantially.
Many later locomotives had installed instead a device called a front-end throttle which placed the throttle in the smokebox, between the superheater and the steam chests. This dramatically improved the responsiveness of the locomotive. It is normally obvious externally that a front-end throttle is installed; there is a crank mounted on the outside of the right hand side of the smokebox, and a throttle rod travelling the whole length of the locomotive to the throttle lever in the cab. The rod is normally in two parts, connected with a 180 degree crank in the middle; the purpose of this is so that thermal expansion of the boiler cannot change the throttle setting. Otherwise, an unattended locomotive's throttle could open and cause a runaway.
The rodding for the conventional, steam dome throttle normally passes through the boiler itself and cannot be seen from the outside.