My opinion on this is that unless you can also have an example forklift operating on a software program that anticipates what a trainee SHOULD be doing, and stops or sounds an error alarm each time something inappropriate is done by the trainee........you aren't actually gaining a thing when going through the process of simulator to real truck.
I can see where a simulator program might possibly be useful for efficiency and productivity training to fine tune or hone the skills of an already certified forklift operator if the simulations run could depict the actual working conditions of a specific employer's operation.
This could be set up to work like a video game wherein the object is to move as much "product" as possible without wrecking anything. Would have to be capable of mimicking warehouse operations, freight dock terminals, rail yard terminals, and don't forget the giants handling containerized cargo on ships, trucks, and rail.
But there are so many variables to contend with, I doubt any gameware author could tailor a program "realistic enough" to be of much use for anything other than "entertainment".