I've often wondered how such interchanged cars were handled in trains of the two railroads.
The D&RG and pre-C&S systems used completely different and incompatible air brake systems.
The D&RG latched onto straight air brakes early on: Locomotive air pump and large tender reservoir increase train line pressure to force air into the cylinders on individual cars to apply the brakes. If the train line were broken (at air hoses), then no air line pressure = no air brakes = train runs away.
After being acquired by the UP, the South Park ditched the Eames vacuum brakes and rapidly embraced automatic air brakes, as early as 1882-83. (The UP built 27-foot freight cars were delivered with automatic air brakes.) Locomotive and its reservoir increase train line pressure, triple valves fill brake reservoirs on cars; when reservoirs fully charged, triple valve bleeds air from cylinders and release the air brakes. To make an air brake application, train line air pressure was decreased by the engineer, and triple valves diverted air from reservoirs to cylinders, applying brakes. If the train line were broken, train line pressure falls to zero = all air brakes applied = entire train stops.
Despite the obvious advantage of automatic air braking, the D&RG stubbornly clung to straight air brakes for 20 years, until forced to change over in 1903, due to federal regulations. Indeed, the D&RG scrapped a couple of thousand 24-foot B&S boxcars rather than converting them, acquiring the 3000 series of 30-foot boxcars, delivered with automatic air brakes in 1903, instead.
So, before 1903, how were South Park/DL&G/early C&S cars handled in D&RG trains and vice versa?
In reading about the early Northern Pacific, as standard gauge freight cars were converted to air brakes, the sides of the house cars were stenciled with "AIR BRAKE", diagonally, to the right of the car door. This allowed the switchmen / brakemen to block all air brake equipped cars to the front of the train, behind the locomotive; all non-air brake cars were blocked at the rear, ahead of the caboose.
Were interchanged C&S and D&RG cars, in the opposite road's trains, handled the same way: At the end of the train, ahead of the caboose? Anyone know for sure?