Granted I have not tried flexi-track on foam, but have secured track with normal track pins . If necessary 1inch long household pins could be used. Once ballasted, and glued then that helps secure track.
If I was bending track to tight radius I would pre-bend the rail, then put it back onto sleepers.
Another advantage of the foam is that it is easier to move track around to test layout before fixing down.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges. http://www.rue-d-etropal.com