Could you both try again with v0.1.27? (requires Factorio 2.1).
I tested it with LCCC (both full and empty) and it behaved just fine.
Normal LCCC -> 40 slots per carriage
SpaceTrain LCCC -> 50 slots per carriage
SpaceTrain accelerates roughly twice as fast as a normal train, even with the additional mass, which seems fair.
What freight (item type) is in your carriages? Maybe these items have exceptional weight? I'm testing with copper ore.