best method and longest lasting....please
I assume we are talking about large scale here. If you live in a non freezing climate, then water proof yellow glue such a Franklin's Titebrond II mixed with water will do a decent job. Wet the marble with water then add the glue/water mix. 25% glue, 75% water should about do it.
If you live in a freezing climate, then applying the crushed marble and packing it down with water and a bit of tamping is about as good as it gets. I have tried a number of diluted glues, including epoxy diluted with acetone. But none of them could stand up to water freezing and expanding. There is a period in the fall when rain or melt water soaked up by the ballast during the day freezes and expands at night, forcing even the best glued grains apart.
I have even tried filling the spaces between the ties with mortar and pressing crushed granite into it. Same result. Adding a bonding agent stuck the mortar permanently to the concrete base, but not to the ballast.
My final comment on the matter ... ARRGHHH.
Jim
A bit extreme, but try using a cement made by DAP or USG. Using it would be a trifle expensive and you will have to anchor your track in the bed. I've used it and it works. I had to transverse 30" of sidewalk. Of course, the better half had me tear the sidewalk up the following year. So, my labors should have waited a year.
The product comes in a tube and is used in securing landscape blocks. You can find it at Lowe's, Home Depot, etc. Usually in the paint department with the adhesives.
Another alternative would be a mortar or sand mix concrete bed. You will have to make sure the old concrete is cleaned and etched, at least have a surface for the new stuff to adhere. Personally, I haven't tried that remedy. But, as they say, it looks good in theory. And now there is no need to worry on my garden layout. No sidewalks.
Bob