Although MTL seems to think these cars are based on an Evans 4750 cf prototype, I'm told that they're actually based on an Evans 4780 cf prototype (built 1977-81).
Here's what MTL has to say about them -
This Evans three bay covered hopper was first built in 1966. This car is a 4750-cuft, three compartment aluminum covered hopper with 100-ton BarberŪ roller bearing trucks. Corrosion resistance and the lightweight of aluminum made it especially suited to the construction of large covered hoppers. The 4750-cuft aluminum covered hopper car offers a payload of 105 tons, nearly 7 profit-tons more than a steel car of comparable size.