Is there a way to have it more simple than checking everywhere with %2 or /2 ?

we can create a boolean

easyMode = bar.level % 2 == 0

to check if it has 12 or 20 wagons in sample Grid and then use this variable instead. what do you say?

a better name than easyMode :).
you acn also create a function to compute the uniqueId/imageId because there are a lot of magic numbers.
Having a new dataset variable in the js could also avoid ceil/floor?