Behemoths (1): Failbird105
Oko Cyklonu (1): UristMcRiley
Colony Trains (1): MoP
So I feel like you missed something in the train proposal, Urist.
"While many were quick to point out the need for tracks before a train goes anywhere, the Colony Trains were designed to move slowly - barely at walking speed in nominal conditions. Conditions that no longer exist. The trains move only at night, and
during the day the Engineering Teams (tethered to the train) build portions of the track to allow progress the next night."
There's also, from the prompt, "It can be assumed that land-based transport was able to cross water with little difficulty, as most seas and oceans were long since frozen over by this point." Polar Express did it, and that world wasn't even deep frozen!
As for my breakdown of our options, keeping the above in mind, I feel like breaking through ice thick enough to support whatever land transports is easier said than done. That said, if the seas are actually sailable, it makes a total of six lanes a bit easier to deal with. The lack of any specifications past "it's a military/fishing/research/colony boat" is a little worrying and seems like quite a, pardon the pun, wide net to cast. Which, also, I dunno how well fishing's gonna go in the environment.
Behemoths are a solid option, but I feel like they're a good and viable choice at any point in the game, and we should get a gauge on the GM's difficulty scale earlier rather than later - especially with 2s getting rerolled, and we're getting design prompts. We
might not get the option to try trains before the end of the prelim turns (I imagine at least two turns will be taken by "gun" and "artillery" or something similar). As designed, they
should aid in every lane.
With the colony trains we give ourselves a solid basis for a lot of things train-related that you never really see in arms races. It also gives us the chance to see if non-combat aspects of our designs will have impact, and if so, how deeply. The colony train gives a nice foundation for a good few technologies we might want to have to make it harder to take us on. The Colony Trains can be easily swapped to troop transports, and it gives us a solid basis and network to build and utilize some fuckhuge artillery pieces. With how slow I imagine overland movement will be, having a massive mobile artillery battery (with spotter balloons?) would be devastating.
EDIT: Oh, and it could be helpful on all lanes, but at worst it'll be useful on 12. On the basis of useful lanes alone, I think the Behemoth or CTs are most viable. On account of the special rules of the prelim turns, I think that narrows it down to the more ambitious option. Plus, it maximizes the revisions we have to fix it should we get hit with a big ol' difficulty modifier.