I believe you are correct; the whistle in question is from Silverton Railroad No. 100. All this talk of re-opening the Silverton Northern engine house, possibly re-laying part of the SN, and maybe mounting No. 100's whistle on the 315 would surely have Otto Mears stirring in his grave....that is, if he had one! (His ashes were scattered on Engineer Mountain, if I recall correcctly)