Finicky Flups

[GrandpaKim 7]

I'm trying to put in a Flup underpass going under a an 8S highway.  This is a long straight stretch of highway.  Very long.  It is nothing but a long dragged highway on flat ground, no ramps, no transitions, nothing and there is no development nearby.  When I try to place the Flup it refused to go in some of the locations I might want it.  It just flips to the next tile, an effect we've all seen.  I would say about 40% of the tiles just won't accept it.  It is not critical where the Flup goes so I can make this happen, but I do wonder what is going on.

SC4 is working perfectly for me after taking advice from these forums.  I have very few mods installed.  NAM, of course.  Bug fix mods, like the Opera House.  Two small lots in the Power menu.

[rsc204 21,804]

When you say flat, how flat is flat, is it actually perfectly flat? Pick the spot you want the underpass, demolish three tiles of RHW, the one where you want the underpass and one tile either side of it. Place a rail stub (just click with rail tool, without dragging) in the middle tile, then place one either side of it to perfectly level the intersection. Now delete the stubs and rebuild the RHW, the FLUPs should place without problem now.

[GrandpaKim 7]

AH!  Okay, like building an interchange, it's best to be perfectly flat.  As you suggest, the area is not perfectly flat but does have the occasional .1 or .2 metre variation.  Thanks rsc204!