I used Scale City Designs dummy couplers to close-couple my Aerotrain cars. The shaft has 2 holes and you can cut it to length. I ended up drilling a hole halfway between the other 2 and tapping it for a 6mm long M3 screw. I applied Locktite to the screw then mounted it to the existing MTH coupler mount. And at $7 for 6 couplers, the price can't be beat.
Bob