Smaller trains! So that you can make smaller cuter stations and more compact spaghetti.
Graphics by Sacredanarchy.

Normal Locomotives are 6x2 entities. That is pretty big!
The new Mini Locomotives are only 2x2!

Normal cargo wagons are also 6x2, which is also big! Sometimes you don't even want that much space.
the new Mini Cargo wagons are only 4x2!

The Mini trains are compatible with the base game wagons and vice versa.