It is true, Western Pacific had repurposed steam locomotive tenders to use as water tenders for the boilers for passenger trains. Three tenders from scrapped 171 class steam locomotives, 174, 175, and 177 were used as auxiliary water tanks. These were renumbered 853, 851, 852 and painted orange and silver to match the motive power.
Information courtesy of Feather River Historical Society and WPRR Railroad Museum Western Pacific Railroad History Online.