Water Tender

Actually, some railroads did indeed use "tank cars" for additional auxiliary water supply for their steam locomotives. The B&O and IC come to mind.

In the modern excursion ere, former locomotive tender were/are utilized as they were available at the time, and looked much better behind the man steam locomotive tend when used in passenger excursion service. An additional factor for NOT using a simple "tank car" for additional water supply in passenger service, would be the trucks of the tank car would NOT have capable of passenger train speeds.

The Steam Railroading Institute that runs the Pere Marquette 1225 has a tank car that they are making into a Aux tender for when they do photo freight excursions.   Not sure where they are on the project.  They do have an old Rock Island locomotive tender that has been converted to a Aux tender and they use on longer excursions now.  

Because, they usually were converted coal/oil tenders.

Early in 1522's fantrip career, a converted tank car was used.

Frankly, a converted tender looks better behind an old chooch.

Rusty

Here is a Pennsy tender that was used as a water car for Reading & Northern 425.

It was replaced by a smaller Taylor tender (previously owned by teh late George Hart) from a Reading Camelback. This one fits on the turntable at Jim Thorpe with 425. The Pennsy tender had to be turned separately.

I think that tenders were preferred because they were available and required little modification. Some railroads removed the coal bin to provide maximum water capacity. They were also used to carry water for steam generators on diesels on long passenger runs.

Tank cars would have to be refitted as water cars and removed from regular service. As Hot Water posted, their trucks may have required replacement as well.

Water cars came into use late in the steam era. Diesels ignored every water plug on the system. Water cars enabled steam locomotives to run longer distances. Surplus tenders filled that need at minimal expense.

Was a new or rebuilt water car made for the SP 4449?

The auxiliary water tender for 4449 was made out of a cab forward tender (4219), which the SP had been using in fire service, up in the Cascade Mountains (south of Eugene). The aux tank was made to accompany the 4449 on the VERY LONG, 1984 New Orleans Worlds Fair Daylight train trip (otherwise known as 'The New Orleans Death March'). The original square top portion of the tender where the oil bunker was located was concerted to all water, and the square corner war rounded off so that the whole aux tank matches the rear water portion of 4449's main tender. The aux tank was also up-graded to roller bearings. 

It sure is nice to have that EXTRA 30,000 gallons of water!