Cooper E-80 is a loading due to railroad locomotives and is the standard for designing railway bridges. You'll need to get the design manual from AREA (American Railway Engineer's Association?) for exact information.
E-80 surcharge has various axle loadings and spacings, but the heaviest is an 80 kip axle load at 5 feet on center. Spread over the rail ties, the vertical surcharge is 1882 psf or 80,000# / (5'LF x 8.5' tie width).
I was taught that E-80 stood for Equivalent 80kN axial load, the maximum allowed load per axle for trucks. As trucks cause the damage to pavements, E-80s are used to design road pavements, at least in South Africa. A car counts a 0 E-80s, while a truck would be 1 (one truck) or 5 (five axles per 18-wheeler), I do not remember, with other vehicles being fractions (or multiples) of E-80s.
The Cooper E-80 surcharge I described above is specified by Conrail, Amtrak, Norfolk Southern, CSX, and others.
The 80,000# axle load acts on a 5 foot axle spacing and 8.5 feet wide ties. Therefore, the railroads are looking for an 1800 to 1882 psf, vertical, strip pressure used with the Boussinesq equation for a RIGID wall.
wpr - Your also asked for "readily accessible reference on the web". Take a look at US Army Corps of Engineers "Railroad Design & Rehabilitation" (free .pdf download) at this link