Hi Ricalls and welcome to Innspiring.com What classification you fall under is likely going to depend on state and local laws. I'm not familiar with Indianna Laws but I'll give you a similar example.
I have cabins (not a stretch to picture them as traincars minus the wheels) that I run as a B&B in NY state. What you call yourself on legal papers does not have to match how you market yourself. The law in this county is that a B&B has no more than 5 bedrooms as part of the house and that an innkeeper must live in the house. Since ours are cabins, the legal health code applied to us is "transient lodging" but that is separate from marketing our approach and even calling ourselves a B&B. We live on the property and serve Breakfast (only to our guests) in our house.
Quite likely your train car cabin idea could play out VERY well if executed right. People love the idea of a B&B but they also love the idea of having their own space. You of course would want to offer ensuite baths for each car as well as heat/ac. Queen or King beds if going for the romantic market (popular) or even bunks (within reason) if you were going for family friendly. The most likely issue you will run into is the health codes related to fire (emergency exits have to be in certain locations and sizes), health codes related to water and septic, and health codes related to serving food.
In Ithaca NY there was a restaurant in an old train station that also had 3 cars outfitted as overnight accommodations. The rooms as far as I know were always booked. Unfortunately the restaurant was a different story and lost money. The three cars were not enough to support the restaurant so the venture collapsed, but it was not due to the lodging in the cars..