Workers can't build RR in certain squares?

[m0nk33]

This is a minor thing that I notice once in a while and I cannot figure out whats going on...

When my workers are standing in certain squares, the option to build Railroad isn't there, even though it should be, and if they move to the next square, the option is there. This will happen even when the two squares are identical ie two plains tiles, each with irrigation and road.

Anyone know why this happens?

[Lord Silvi]

One possible reason may be:
The squares in question have already RR, but they have no connection to other RR-squares. In this situation it is hard to see the very small RR-graphic.