[Rollo]

I have also tested this now. The drop troops ability doesn't seem to affect the troop transport (TT) behavior. So you don't actually have to put that in. Putting troops and TT into the construction file is sufficient.

AJC, are you sure the TT was on its way to a target world and not trying to join a fleet? Did you check its orders? I have never seen this behavior of TTs trying to invade a planet alone, only as part of a fleet.
Usually the problem with TTs is that they sit in orbit doing nothing until some time they will join a fleet, if one passes by. If you see a TT leaving orbit, please check its orders. It should say "Join fleet x". After the TT joined the fleet the fleet orders automatically change to "Capture planet". Depending on how many fleets you have moving around and given that the TT is not waiting in some backwater world, this can work quite well or nothing at all. I have seen the Vikings capture planets as early as turn 35 and as late as... well, never.

Rollo