You may never forgive me for this Rich, but from a Google books search, the earliest reference to "lash up" is at least back to 1968, "Pennsy power: steam and electric locomotives of the Pennsylvania Railroad, 1900-1957:"
This practice has been dubbed a lash-up by railroaders, because of the extra wiring necessary to achieve control over dissimilar locomotives.
And also in 1968, "Bulletin of the National Railway Historical Society"
But the sight of a motive- power lash-up from two or three different roads is for the benefit of the lines involved, not primarily to gladden the railfan enjoying the sight of a mixture of locomotive liveries.
From 1969, "Business Week":
From the mainland, a dirt causeway will bring out the unit trains— Canadian Pacific lashups of as many as 13 locomotives, five or more run by remote control , powering 100 to 115 coal-hauling cars.
Prior to that date, "lash up" seems only be used for something improvised or jury-rigged.