This is a train for rainbow blocks. AFAIK there are two main sizes of the block, larger ones with length of 140mm and smaller ones with length of 100mm.

I make a model for both of them, but print-tested only for the larger one. The tolerances are generous and all edges are chamfered for comfortable handling during play.

There is also and SCAD file for customization.

Additional material: per train wagon: 4x M8 washers 4x M4 wide washer (DIN 9021) 4x M4x8 screws

I recommend to print the main body with brim AND support to have good adhesion of the sticking out parts