running air brake test

[GM1361]

The following instructions are an extract from Alaska Railroad Air brake and Train Handling rule book.

My question is why running air brake test is not required ( according to the highlighted part) when cars are cut off from the rear end of the train only?

61.8 Running Air Brake Test

Conduct a running air brake test of all passenger trains when:
• The train leaves the initial terminal.
• Locomotive, engine or train crew, or operating ends have been
changed.
• Any angle cocks or cutout cocks have been closed. However, the
running test is not required when cars are cut off from the rear
end of the train only.
• The train has struck debris on the track.

[jeffhergert]

The test is in part to verify that you have air brake continuity through the entire brake pipe, from front to the last car in the train.  If the train picked up or set out cars in the middle of the train, an anglecock might have been left closed after the work was done, blocking the brake pipe from there to the end of the train.  The running test physically gives an indication that you have brakes working on the train.

If the only work being done is cutting off car(s) at the rear of the train, you aren't affecting the brake pipe between the engines and the (new) last car of the train.  Since the continuity hasn't been compromised, it doesn't need to be tested again.

Jeff