The RailKing "madison" type, heavyweight cars are 60' cars.
They've done the Santa Fe/AT&SF cars in a few runs, and they should look ok together. It appears you have items #30-6907, #30-6908 & #30-6909.
You could look to add on to what you have with:
30-6955 (4-car set), 30-6956 (2-car add-on), 30-6957 (add-on coach)
You could also consider Pullman Company versions to mix in:
30-69016 (4-car set), 30-69017 (2-car add-on), 30-69018 (add-on coach)
I always thought these RailKing heavyweight cars were nice and a great hobby value.