Definition of "compartmentalised"

This was the first long train journey I ever took, and the details remain in my memory as sharply as if they had taken place last week. Hopping on the Circle Line to King's Cross, boarding the compartmentalised train, the kindly ticket inspectors, and every station where the Grimsby express stopped (in those days it didn't go through to Cleethorpes).

2021 July 28, Christian Wolmar, “Forgotten by the railways, but ripe for the exploring”, in RAIL, number 936, page 34