Gerhard, Interesting !? However, can they just print the Inner 'concrete' (between rails) and Outside the rail?? Outside the rail would be a small width, ~1/2 meter or less, in contact with the rail. Approximately 0.1"/2.5mm.
The CAD model should be able to remove all the large pieces, support and interlock. Results would be trimmed to lenght.
Jeff
SF Bay Area Z
a.k.a. 'The BAZ BoyZ'
Jeff Merrill BAZ BoyZ