Nards,
it sounds like you need to make a transition car.
Replace the truck & coupler on one of your older
cars with a modern truck & coupler,  If you don't
have a older car to use, change out the truck &
coupler on one your modern cars with a older truck & coupler.
I do this with my N scale trains when  cars have the
old style hook & horn coupler's.
Also have a few Large Scale transitions cars &
an HLW Mack with a hook & loop on one end
 & a kadee coupler on the other.

(HLW- Hartland Locomotive Works- American made 1:24 SCALE TRAINS)