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!, and also don't have a gap between them when in the station.

The Mini trains are not super compatible with the base game wagons and vice versa, there might be some alignment issues with inserters and pumps and such. So it is recommended to only use the Mini trains with Mini wagons and Normal Locomotives with Normal wagons.