It feels like a bit of a waste to bury such nice trackwork under concrete! But nonetheless I look forward to seeing how you tackle it.
The real thing was done in two pours - the first pour established a base slab onto which the track was bolted, the second pour brought the concrete surface level with the railhead. The second pour was shuttered on both sides of each rail, and so far I've yet to see anyone model this detail. The idea was that the track could be removed and replaced since the bolts were still accessible. I think you could probably do it with a first pour to fill the gaps between the sleepers, and then glue some styrene angle section either side of the rail up against the chairs to act as a form for the second pour. If you wanted to be super realistic, use some fine sand to partially fill the gaps. In less travelled bits plants and grasses would grow in the sand, another detail perhaps.
Another detail I never see on model clay works is hose pipes for hosing down the concrete after loading. The loading edge wall would have spigots and coiled up hoses hanging on hooks. You can see what I mean in this photo on the right hand side just below Elaine.