I use limestone “crusher fines” as we call it in these parts
Re-ballast pics from 2011
Original pics from 2006/2007
If I’m out of line and off topic I apologize
i just use decomposed granite that is sluffing off a hillside a couple blocks away. Add more ballast as mother nature washes it away.
#1 chicken grit and top-up when/where required !
then use the RBR backshop highly engineered channeled 2x4 PT block attached to 1/4 sheet sander with chunk of memory foam and tire remnant to help (vibrate) the little 'uns to settle in and theoritically lock into place…
under the track is 3-6" of 7mm(?) Gyra, tamped down to grade
doug c