Cooper E80 simulates the live load from a train, usually on a bridge. It is a series of point loads (simulating locomotive wheels) followed by uniform distibuted load of 8 kips per linear foot of track (the total load on two rails to simulate railcars). As is so often the case, it is more complicated that this. There are factors for impact, sideways imbalances, longitudinal forces, multiple tracks, different values for reaction, shear & moment that all differ depending on span length.
Most civil engineering handbooks, especially older editions, will give give some coverage to Cooper loading.
A final note, the "Standard" Cooper train is an E10. Cooper E80 is eight times larger and is a reasonable approximation for the requirements of a main railway line.