Not an expert on Water Cars.
CP had old tenders used as Auxiliary Water Cars for use behind a 'working' tender.
CP also had Water Cars which were tank cars with Dome cut off, a wood platform with rails, and a hatch where dome used to be. These were also used to bring water to outfit cars in remote locations where water might not be available.
( I once as a side duty, did not get paid, had to 'fill' Water Cars in Gang Service WITH A GARDEN HOSE over a weekend and it took forever, and affected water pressure for showers, etc.
When full, it would overflow until shut off and hose removed.
A Tank Car Type was usually abt 8000 Gallons, and some had oil company logos still on them.
Some Water Cars were modern 1930's steam tenders, some still in full striping from Psgr Service, and a six digit OCS number applied. and they never filled, if empty, over a weekend.
Other true tender water cars still had COAL in them, and stoker screw just just off. And a coupler, grab irons and a Hand Brake .)
Adding a water car to a locomotive w/tender added 'Delay' a nasty word, often.
The second water car/tender had to be spotted up, a second move, then filled. Thats why some track side water tanks were 100,000 gallons if water supply was ONLY 1000 Gallons an hour from source. The 'Ball' on top of mast was attached to a float inside, and Engineers, Pumpmen, etc could see level in side by ball.
Trackside water tanks had to be heated, a Briquette stove for the purpose in base, it's chimney passing thru water space in tank above.
Some tanks leaked, and you could stand under leak in summer and cool off.
Agent or Section Foreman looked after tank stove. Bruce would know if Agent got paid for tank heater.
With an auxiliary tender, a locomotive might be too long for turntable, so locomotive and tender, then auxiliary tender would have to be turned, as water connections usually on only ONE 1 end of auxiliary car. Ditto being too long for inside a Roundhouse to keep warm when not under steam.
Tenders could freeze, and steam could be sent back thru Injectors or piping to water. Oil tenders had tank heater to keep oil at optimum operating temps, as various oils had to be kept warmer than others for Firing, or would 'Gel' if too cool. The top of intake for oil to locomotive burner was raised up off floor of oil bunker and had a screen on top to prevent inhalation of rag or waste that had fallen in thru filler hatch atop.
The condensate from steam coils would collect in bottom of oil tank and have to be drained regularly. Some oil fuel was messy and could contain water from refining process, as it was often a 'Garbage' fuel and "CHEAP,' another Operating Word in use much of the time similar to the word 'SAFETY'
I do NOT know if there were steam lines MUd to a water car for winter? Only certain locomotives would be Plumbed to accommodate a water car connected behind.
I assume a water car was just drained when not needed in cold weather, and left outside.
Filling TWO cars, tender/water car took more time than filling ONE larger tender applied, and a larger tender got rid of the extra plumbing and may well have not increased whole locomotive length for turntable/roundhouse.
I have not seen any water cars added to any 'Big' steam locomotives on CPR, nor used in passenger service on Main Line type service.
Not all water was really good for locomotive service, so, with an extra tank, a locomotive could travel further, and pass up poorer quality tanks re boiler scaling and treatment.
If only 1 or 2 trains a week, the Company might well give every train a water car, and eliminate 'Bad' water tanks, decreasing costs, and stops?? CP put oil burners on one long run to obviate need for coaling en route.
Water Cars could also be used in Fire Service in Summer and some so assigned w/ pumps and hoses.
Much More could be added.
This may be a start??
I have to go to Starbucks, the snow has stopped and now okay to walk.
I will look for CP water car photos when I get back, as they are out there.