Ive used a mixture of methods and here gos:
Concrete
I just take off an inch of turf, no need to go very deep, as its not structural
You just make a mold/ shuttering and pour away
You get custom shape trackbed.
Its very permanent and when the track is screwed in via rawlplugs its doesnt move.
I used a 5:1 sand cement mix aka a weak mortar mix,
loose Ballast
You need about 40 mm ballast depth,
looks good,
easy to adjust to iron out any bumps,
make sure you put down a weed membrane to keep dirt and ballast seperate
Ideal if you like to rearrange track constantly
Raised decking boards
Nice way of making raised railway,
The only problem I had was mixing different sorts of timber for the trackbed, as the different thickness caused problems.
I also used insuffuicient number of supports that caused sagging in places.
Another method is if you want to make a bridge, use 2 lengths of metal L section, with a plywood deck, the metal takes the weight, and you can build the cosmetic bridge frames another day.
Any more questions please ask, and dont be afraid to try something and then rip it out if its not right.
Dan