"cleaned his clock" Antedating to 1946

[Douglas Wilson]

>     ... why would bringing a train to a sudden stop
> by setting the air brakes be described in railroad lingo as "clean the
> clock"
> or "wipe the gauge"?  What exactly is going on here?

An expert on old locomotives could answer better than I can, but I naively
imagine the needle on some clock-like gauge suddenly sweeping across the
gauge face like a windshield wiper. The gauge could be something
functionally equivalent to a speedometer or tachometer, or perhaps it
could be recording pressure or something else which suddenly changes on
rapid braking.

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