Could the mini trains be added in a similar fashion, integrated in the progression?
(https://mods.factorio.com/mod/Mini_Trains)
Perhaps the smallest as the first train, but also include small equivalents of the other trains, to give an option for more compact train stations at the cost of acceleration, speed, and cargo capacity in small wagons.
I understand it would require graphics for the 'later equivalent small locomotives' though.

Also, is it compatible with the mods that change how trains accelerate and brake?
Like https://mods.factorio.com/mod/OverloadedTrains11 or https://mods.factorio.com/mod/TrainSpeeds ?

Those look really cool! I've never used/seen that mod before. It would also be like having access to a pump wagon. Though I'm not sure where that would fit in the balancing for me personally, so i'm going to pass on adding it for the team being, though will bear it in mind for future updates.

I would assume a mod changing braking and acceleration should overwrite any base stats that are set by my mods.