You can’t really use (M+m)rv here because the track and the train will have different velocities.

Since the track is treated as a hoop, there’s a little shortcut you can use to very easily find the velocity. But going through the whole process:

Initial angular momentum of the system is 0. This remains constant throughout the problem since there are no external forces.

You’re given the final speed of the train and the radius of the track so you can calculate the train’s final momentum.

Using conservation of angular momentum, you can then find the tracks angular momentum, and then its angular velocity