I am thinking of using Rails to attach fishing rod holders and other accessories to my kayak.

I see there are multiple brands and diy solutions out there (yak attack, rail blaza etc)

Question is, is there a standard ? i.e. is there specific sets of accessories that attach to specific rails only ? are the inner size of the rails (i.e. the slot bit) different between the brands ?

Or are they all the same size and the difference between the brands is just quality of the rail/size/no. of screws etc ? i.e. the inner slot bit is a standard bolt size or something ?

Dont want to order and install a rail only to find that I bought the "Apple" of rails and can only use expensive accessories.

I think you might be talking about tracks? Track are awesome for accessories but not very good to mount rod holders on, in saying this i had 2 rod holders on my front tracks on my old yak however these were set for freerun (no drag), having rod holders on track mounts is asking for trouble imo, however if your talking about hobie H rail then there are some very good mounts available for them.

Tracks are or should be made to a standard, however i forget the sizes, scotty and railblaza both have rail mounts, however railblaza have a better range of accessories available.