These services are not always proprietary. That is; Plasser and Theurer machinery is used by railroad contractors that are hired to examine geometry and grind rail lines. The UP , which is now SP, had some yellow units I saw in Idaho around a decade ago.
There are a bunch of model railroad models of this stuff. The ready-to-run models all seems to come in some high visibility yellow/orange. The German's make a self propelled abrasive track cleaner P&T unit(s), with a mating unit that carries an onboard vacuum cleaner in- 'HO' scale.
You might ask this question on YAHOO's "MOW" group, as someone there probably knows the answer to it. There is also the Southern Pacific Historical Society, and the equipment manufacturer's WWWs , where you can send them an Email , and enquire if they sold machinery to Southern Pacific.
Good-Luck, PJB
