My son received some new HO train cars for Christmas but the couplers are not compatible with his current train.  The new coupler style is an EZ Mate.  The old ones are horn-hook(?) style couplers.  I am trying to figure out what to do to make it work.  My first idea was to swap couplers on one old and one new car so the new cars could connect at that point.  That did not work because the new couplers are quite different in size and shape from the old couplers and will not fit on the old wheel trucks.  The new cars are also body mounted couplers whereas the old ones have the couplers on the trucks.  I am open to replacing all the old couplers so they are all alike but do not know what to purchase.  This must be a common problem for many with old and new train cars.  How should I best solve it?  Thanks.