Use of Copper Tubing on Engines

[nw-mailing-list at nwhs.org]

I know we have some Motive Power people on the List, so perhaps someone can answer this question...

When did railroads begin to use copper tubing for air brake piping in engine cabs?  (It definitely pre-dated all the backhead photos in the N&W/VTU photo archives !)

-- abram burnett
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