Interesting question... and sounds like Lionel hasn't standardized its approach here. The DD35 dummy units are equipped with electrocouplers, which allows them to participate in full function MU configurations. Same thing can be said for the Legacy Dash-9 dummy I got to complement the NS TankTrain set from 2008/2009.
If the dummy unit doesn't at least have a rear electrocoupler, then it really can't be the last unit in a multi-unit configuration. Not good planning on Lionel's part -- especially for MSRP of $240. Essentially, you're getting a very expensive boxcar! Sounds like the price continues to go up (the MSRP on the NS dummy Dash-9 was $220 vs. $240 for a GP-9 dummy today), but features are short-cutted along the way.
David
P.S. I was toying with the idea of getting the CN dummy GP-9. But I may rethink that if it doesn't come equipped with an electro-coupler. Thanks for bringing this to my attention now!