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.