This is a bit of a weird one, and sorry for the length!...

I'm making a birthday cake for my niece, who's turning 3. Every year for her and her brother I make I 'train cake,' using a great pan by NordicWare that has 9 wells of train cars--engine, caboose, coal,, logging, etc.--each holding about the same amount of batter as about 2 cupcakes. In the past I've decorated them with candy and frosting, then created a 'scene' on a cake board (about 18x22, give or take a bit). Winter wonderland, enchanted forest, etc, using buttercream and then a variety of things to make flora, fauna, lake, hills, rocks, etc,  train tracks and then the train cars themselves. 

Their birthdays have become synonymous with also getting a train cake, they look forward to it, and whoever's birthday it is gets to decide details like flavor, colors, etc. 

My niece was recently introduced to Little Mermaid--she doesn't know the story or other characters, just what Ariel looks like, and that she's is a mermaid and a princess. So she wants an Ariel/Mermaid train cake.

I'm thinking that means and Underwater Train Cake. Shells, coral reefs, seaweed, animal life, etc. I've done tons of research about mermaid/ocean/beach cakes, so I have a sense of the elements I'll use.

I've also figured out that this year, I'm going to make the train itself out of chocolate--not chocolate cake, but chocolate (tempered), using the pan as a mold and making the cars hollow. I've practiced and it looks fantastic, if I do say so myself.

Now I'm left with trying to figure out the structure of the cake itself: what shape(s)? where is the train going to go--on the cake board circling the cake, or the cake itself? Two tiers, 1 big circle, or square/rectangle?

If anyone has any ideas, specifically about the overall structure of the cake (for example, not suggesting I include sea shells, but instead cake pan shapes/sizes), I would be so grateful. Maybe I should somehow do a land/water design, with the train on dry land? The problem is, most of my elements are aquatic, like fish, seashells, crabs, seahorses, etc and I've practiced making ocean plants and coral. 

The cake should feed about 50 people, on the higher end.

So, if you feel like a bit of a challenge, put on your thinking cap and let me know what you've come up with! 

Thanks so much in advance!

Note: I'm a medium-advanced amateur/hobbyist baker. I was planning on using (besides cake), probably buttercream (butter-based, from a recipe an instructor from a Craftsy cake decorating class), modeling chocolate, some fondant, and although I'm not an expert, I'm proficient in making isomalt deco ratings (fantastic effect for coral, and it's easy!)

Sorry this is so long, just wanted to give you  the full picture.

Thanks again!

I can't comment on sizes as I have no idea the size of the train, but if you did a 2 tier round.

Something like this

http://lynscakeart.blogspot.com.au/2010/09/train-tracks-and-tunnel.html

but have the tunnel border as a sandcastle,

something like this

http://www.istockphoto.com/vector/sandcastle-9574960

Have the train incorporated with the tunnel and the tiers decorated underwater/mermaid theme.

What about making a round cake with a train track big enough to hold all of your chocolate trains circling the top perimeter of the cake with water in the middle like this:

http://www.cakecentral.com/g/i/1150579/14-inch-round-single-layer-covered-in-buttercream-frosting-plastic-track-and-trains-water-using-blue-sugar-sprinkles-ducks-made-of-fondant-covered-bridge-tunnel-out-of-cardboard-and-then-covered-with-frosting-attached-with-toothpicks/

... and instead of ducks, have the mermaids tail coming out of the water like this:

http://www.cakecentral.com/g/i/3105568/ariel-the-little-mermaid-cake-and-cupcake-tower-made-for-a-sweet-little-girl-s-5th-birthday-i-was-inspired-by-the-lovely-cake-made-by-angela-angel-food-4-here-on-cc-and-she-was-gracious-enough-to-allow-me-to-use-her-design-i-also-made-40-fondant/

Then make the side of the cake blue so you can incorporate all of your aquatic elements.  So basically you could use the mermaid cake as your model, but have a train circling the top part of the cake like the first picture.  I hope this makes sense.

I also like Sanalee's ideas.