Hm I think that moulded Rail would need some reinforcement to be worthwhile, then on tha basis of smallness of section differences if materials would caous break-up if say Brass Wire were used or Concrete Cancer should Steel be used. Mad idea doomed to failure I believe.
If you're casting exposed metal into concrete then it will need to be non-ferrous
It would also need some sort of lip / edge / tang to stop it sliding out. Stainless is an option, aluminium has been shown to have reasonable wear properties, so an unequal angle would do the trick, or good ole' brass rail.
The difficulty even with code 332 rail is that you are entirely reliant on the sand / cement laitence within the concrete mix for gripping the rail section - therefore you've got to make sure that the mix is well tamped around the metal section although you could insert a brass screw into the foot of he rail every so often
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The drawback that I see is that once you have your concrete sections, you've got to lay them on something, and concrete is unforgiving, so the joints will be under greater stress.
Many of us rely on the inherent flexibility of plastic sleepered track to take out the minor undulations in the sub-base
'twould be an interesting experiment, but you need to have a pretty good idea of the civil engineering support to it before you start ....................... I think