I would like it if when you were driving the cyclops, you had a small side map that showed the sub in relation to its surroundings, because it seems to bump into things a lot.
The trains might be useful if you always want it to travel along a set path, and it would probably be much faster and roomier then the seamoth, with super scenic views!
What if you had hardmode, without the ship, and then when playing normally you find out that the sunken ship was your ship from hardmode, degraded over many years?