when i thought id heard it all.....?

Well, here's my tuppence-worth (two cents?) as regards British practise!

Freezing of water in locomotive tenders was never a problem in the UK. When locomotives stood overnight in prolonged cold weather small fires could be lit beneath the tenders, but the possibility of several thousand gallons of water freezing solid overnight was remote. On a locomotive in service, the continual movement of the water would surely be enough to stop freezing.

Warming the feed water was a different matter all together, and for a couple of reasons. Firstly, it soon became apparent that feeding cold water into a boiler did not do it a great deal of good, due to the thermal stresses to the plates, tubes and joints. The invention by the French of the Giffard steam injector around 1860 replaced the axle-driven feed pumps, and eased the problem in that the injector warmed the feed water. Nevertheless, it became more common to place the feed clack-valves on the sides of the boiler barrel, and to take the feed through a length of piping inside the boiler before discharging into it, thus giving the water a chance to heat up. Top-feed later became more-or-less standard on larger locomotives, in which the feed clacks were placed on top of the boiler. Here, the feed water was discharged into the steam space, falling onto a series of trays to heat it.

The second point in favour of feed water heating was to increase efficiency by using otherwise waste heat to warm the feed water. In the Victorian period (i.e., pre-1900) several railways fitted a pipe from the overflow pipe of the injector that passed through the tender. When it was desirable to prevent the safety-valves lifting, the steam valve to the injector was opened, and steam from the boiler then passed through the overflow pipe and through the tender, thus warming the water.

Move-ambitious feed-water heating systems were experimented with in the 20th century. The problem here was that ordinary locomotive injectors won't work with hot water, and so mechanical feed pumps had to be used instead. One of the most successful was the French ACFI system, which was fitted to a number of locomotives on the London & North Eastern Railway in the 1930s. The feed water was heated in heat-exchanging drums mounted on top of the boiler. However, like all such systems, time showed that the increased first cost and additional maintenance charges out-weighed the other advantages, and the gear was removed.

As to additives to the water, no form of anti-freeze was ever thought necessary in the UK, so far as I am aware. However, water-softening treatment became common in the UK from the 1920s: many areas of Britain have hard water, and this caused rapid scaling of boilers, necessitating more-frequent 'washout days' for engines compared with soft water areas. The water was pretreated to soften it before being supplied to the locomotives, but it took some experiment to find the correct mix of chemicals. If they got into the boiers in too-high a concentration, they did more harm than good, dissolving the beneficial small build up of scale in pipe and plate joints that otherwise helped to keep them steam-tight. The Southern Railway adopted the French (again!) TIA treatment system, consisting of chemical pellets that were added to the water in the tender or locomotive tanks directly. This lasted until the the end of steam on the Southern Region of British Railways, in 1967.

I'll throw my 2 cents in here(as usual lol).I recently got to be the pilot engineer on UP's 844 steam excursion train on a portion of it's trip through New Mexico in November.They kept testing the feed water going to the boiler because they said the water quality was very bad for engine.That led to some conversation about the system.Now keep in mind that my knowledge of steam engines is quite lacking.But I seem to recall them saying that the tender water was at ambient temperature and the exhausting steam was used to heat the incoming water.The reason being that dumping cold water into the boiler would reduce the engines efficiency.I didn't ask them where in the system that took place but I assume that it happened after the water left the tender.It seems that somewhere I have read that some tenders had steam pumped back to them though.I could be wrong though.I'm sure Bob(Samurai) will be able to explain it much better than I.

Draw!

"In a closed feedwater heater the feedwater is run through a small pipe which is contained in a chamber of steam. Condensed steam (I'm pretty sure that is hot water) is collected and piped back into the tender. One disadvantage of closed feedwater heaters was that the additional small copper piping was prone to leaks. The Elesco and Coffin feedwater heaters are examples of this type. The company name "Elesco" was taken from "Locomotive Superheater Company". "LSCo" is pronounced "El" "Es" "Co" or "Elesco".

Tried to trip up ol' Samurai, hunh?

So, now, you've heard it all.

Edit: I forgot to add, not all locos had feed water heaters, and the Worthington BL was the most common, which is an open system but the exhausted steam contained lubricator oil and their drawback was that oil mixed with the water going back into the boiler.