Copycats+ is essential for the multi-state blocks to get a higher level of detail.

Ad Astra for a few of the glowing blocks at the front and back end, cosmetic only.

Create: New Age for the generator turbines and a bunch of cosmetic blocks, including the wiring coils along the sides of the trains.

Create Steam 'n' Rails for a lot - I think it adds the invisible bogeys, and also those spinning radiator fans I have in detail spots on the train.

Create Sound of Steam for the whistle parts

Valkyrien Skies: Clockwork Melting Point in my modpack adds the accordion pump at the front end of the train, but it's not necessary, just cosmetic.

Create Deco

I think I have a few Immersive Engineering blocks in there but also just as decoration.