eu01,
Do you know where Modlin stands with its rail link?
Without it, even Wizzair might see they can get better yields at WAW, although I note FR have stayed out, even with their move to more 'main' airports.
Yet the nearest rail line is 5k away. In the UK, the link to GLA got abandoned after costs headed towards £200m, and that was just for about 1k. On a different not, Poland recently abandoned its high speed line.
The plans called for a spur and a station underground, hence high build costs even if the cost of infra in Poland is much lower than the UK. I can think of few genuine low cost airports with stations onsite, unless a line happens to be passing by (e.g. PIK), or they grow to a size when these costs can be justified (e.g. STN).
I don't think either apply at EPMO.