Interesting, those die cast cars must be really heavy to require double heading a Legacy engine. Any of my Legacy engines will easily pull a 30 car freight up 2.4% grades. Converted AM engines are a different story. With the possible exception of the Northerns none of my TMCC AM engines will pull 20 cars up a 2.4% grade, a few of them either slip or stall with just 10 cars. I could see needing to double or even triple head the American Models TMCC engines.
I have some AM engines with scale wheels, no traction tires. One diesel will not start a 6 car passenger train on level track due to wheel slip. However, double headed, two scale diesels will pull 9 passenger cars up the 2.4% grade. Adding the second engine appears to more than double the pulling power.