I am trying to build avenue underpass under the rail.
Seems either I can't see that puzzle piece or it is not there
No, I don't believe it is there. Sorry. I have, actually, wanted many more of those flups but they dont make a lot of them. So far it is only under the road network and maybe another.
Hope I helped. :)
Alex
Here's the thing:
All FLUP pieces, except the entrance/exit pieces, were designed to work with Street, GLR, Road, OWR, AVE, RHW, and even the Tram-in-AVE pieces. You just need to put two FLUP-under-Rail pieces. That way, only one piece is needed for every network, and not one piece per network. Saves us from doing more work.