"formed" in "This train is formed of four carriages"

Scotrail platform announcements use the phrase, "formed of four carriages". "formed", appears excessive compared to say, "this train has four carriages."

Example audio:

• 3 carriages: https://youtu.be/VtzW4oFSCaw?t=20
• 4 carriages: https://youtu.be/VtzW4oFSCaw?t=103
• 6 carriages: https://youtu.be/VtzW4oFSCaw?t=59

Can someone explain why such usage, when brevity should be more important?

Platforms, especially at a terminus, can be occupied by a long train of multiple carriages which might be split before departing. This can happen when a long, busy train (say an inbound rush hour service) arrives at a platform and then is split into smaller trains for outbound destinations.

Therefore, it's possible to hear an announcement that a service is "formed of the front four carriages at platform X", which means only those four carriages will depart. The remaining carriages will sit at the platform to form a different train. In the case of "formed of all four carriages", it would mean that only four carriages were on a given platform and all of them would form the departing service.

This style of announcement intended to prevent passengers joining carriages that won't be part of the departing service.