Short googling did not yield any special hsitoric feedwater plants for steam trains. This document by Altair states:

As the years passed, more efficient feed water treatments developed, boiler metal surfaces were maintained free from scale, and the problem of oxygen corrosion became more pronounced. This corrosion along with a trend to higher boiler pressures and the resultant increase in the temperature of the boiler water, highlighted the need for more efficient deaeration equipment. Oxygen attack on boiler metal is accelerated with increased temperatures.
In the early 1920's the first open feed water heater was designed to specifically to remove dissolved gases. This initial design was a counter-flow tray deaerator with a re-boiler coil arranged in the storage section, and an internal vent condenser water box.

Sp it appears that the first dearator was built in the 1920s. So how was corrsion prevented before? Water, unless it's fresh rainwater, contains dissolved minerals - water hardness. When boiling, these precipitate and form a scale layer. While one does not want this scale layer - worse thermal conduction, eventually closes pipes etc, it is a somewhat effective corrosion protection. Altair hint at this when they mention that the first dearator was designed after feedwater treaatment became better.

However, this is not conclusive proof that there was no daeration pre-1900. This could be shown by a survey of train yards etc, if these had any deaeration equipment. Dearation equipment would be especially useful when the feedwater source is mostly soft surface water.

Short googling did not yield any special hsitoric feedwater plants for steam trains. This document by Altair states:

As the years passed, more efficient feed water treatments developed, boiler metal surfaces were maintained free from scale, and the problem of oxygen corrosion became more pronounced. This corrosion along with a trend to higher boiler pressures and the resultant increase in the temperature of the boiler water, highlighted the need for more efficient deaeration equipment. Oxygen attack on boiler metal is accelerated with increased temperatures.
In the early 1920's the first open feed water heater was designed to specifically to remove dissolved gases. This initial design was a counter-flow tray deaerator with a re-boiler coil arranged in the storage section, and an internal vent condenser water box.

Sp it appears that the first dearator was built in the 1920s. So how was corrsion prevented before? Water, unless it's fresh rainwater, contains dissolved minerals - water hardness. When boiling, these precipitate and form a scale layer. While one does not want this scale layer - worse thermal conduction, eventually closes pipes etc, it is a somewhat effective corrosion protection. Altair hint at this when they mention that the first dearator was designed after feedwater treaatment became better.

However, this is not conclusive proof that there was no daeration pre-1900. This could be shown by a survey of train yards etc, if these had any deaeration equipment. Dearation equipment would be especially useful when the feedwater source is mostly soft surface water.

ETA: Commentors ask for a source for the claim that scale serves as a corrosion protection. German wikipedia on Kesselstein - Scale - has this to say:

In 19. and 20th century, boilers were mostly operated without water treatment. Scale had to removed mechanically by Kesselklopfer ("Boiler-bangers"), using pointed hammers. The specific heat load of boilers then was lower and the boilers had no critical areas in regard to scale, so this way of operation was possible. Modern boilers with effiencies would be damaged quickly by scale, so feed water treatment is mandated in industry codes.