My understanding was also that those rail wagons could take up to 25 tonnes, which is most armoured vehicles up to Pz IV and vehicles built on Pz IV chassis. Many pictures exist to prove this also.
The Trojca book (Panzerzug BP42/44) advises that the armoured wagons were built on Ommr Linz rail wagons, & weighed around 32 tonnes each. Possibly Trojca has it wrong, as with overloading a truck, overloading a rail wagon is not a sensible decision. Though they look like thickly armoured boxes, the Panzerzug wagons were relatively lightly armoured, compared to contemporary tanks. Thicker armour was desirable, but could not be added as the load limit prevented it.
Good luck with your casting, don't worry about me, make your own train
I was just curious if you were going to make a commercial quantity as I'm sure you'd sell them.
I have plenty of work to do on the rest of the BP-44 & have a russian armoured train heading my way.