私家車無拉手掣溜後就見得多，成抽EMU溜後會點呢?

I'm guessing the Japanese trains uses the same concept of Main reservoir (MR) and Brake Pipe air (BP).

Looks like they took the parking brakes off after they connected the pipes, but they didn't expect the coupler to lose connection.

This could have been catastrophic if it ran away.