I simply am curious about this, how much is made of metal? And what kind of metal? Brass? Steel?

Also, what parts are in plastic?

Only the most important parts.

I ask because "The running gear with the main frame and locomotive body are constructed of die-cast zinc. The locomotive has many separately applied parts of centrifugal cast brass" [1] (55681) is not very clear to me, and the spare parts list doesn't help any.

I guess that if the skin of the two end parts and the driver's cab were in metal Märklin would say something about "brass sheet metal". So I assume what we basically see is painted(?) plastic?

[1] https://www.maerklin.de/...s/details/article/55681/