I'm reexperiencing this game from my childhood. Having a lot of fun!
I've now unlocked the Manley-Morel DMU (Diesel) and can't for the life of me figure out how to configure the train with two cars.

In the "New Rail Vehicles" dialog the train is pictured with two cars, 38 + 38 = 76 passengers. So when I buy it I get the first car, but buying a second one just creates a new train for 38 passengers. I've tried combining them into one train by dragging the second one on top of the first one, but it's not working.
I suspect this is just me missing something silly. I've tried searching for this, but haven't found anything.
I'm running with TTDPatch 2.0 rev 1 if that makes a difference.