Lee, in the case of your photo, someone in charge might not be in charge for much longer. Rational thinking would have the use of a smaller class of power that had known operating clearances on the line. Prudence should have dictated the placement of several freightcars between the Clearance Gauge and the Locomotive pushing for stopping purposes.
In the case of the D&RGW picture referred to in the first post, it is the clearances limits of the load being recorded, not those of the Locomotives which had been in service for many years and that the dimensions were known. If in the case of a Tunnel bulge or movement or the introduction of larger power, then the Gauge should be pushed as above.