Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I asked Google and this is what they said:

"In addition to the commonly used brass boilers, some Falk toy steam engines did incorporate steel into their boilers.

While brass was a popular and cost-effective choice for early toy steam engines, steel was also used, particularly in later models or specific designs where its strength and durability were desired.

Steel boilers, though more expensive to manufacture than brass, offered increased strength and could potentially provide better support for other engine components."