This is the 11680th most frequent Russian word.
забайкальском
"Забайкальском" refers to something related to the Transbaikal region (beyond Lake Baikal) in Russia.
In this sentence, 'забайкальское' serves as an adjective describing the plateau, indicating its geographical location related to the region of Transbaikalia.
Забайкальское плато является частью Восточной Сибири.
The Transbaikalian Plateau is part of Eastern Siberia.
Here, 'забайкальским' is used as an adjective to modify 'лесам и горам', conveying that the forests and mountains are situated in the Transbaikalian area.
Я путешествовал по забайкальским лесам и горам.
I traveled through the forests and mountains in Transbaikalia.
Here, 'забайкальском' acts as an adjective modifying 'участке железной дороги', denoting that this section pertains to Transbaikalia.